Effects of Lactational Lead Exposure on Infant Development
Section A: No safe level of lead exposure; exposures to lead in alternative infant feedings
Section B: (Summary): Sources of lead in women, and therefore in breast milk, include occupational and hobby exposures, cosmetics, common foods, local and regional industrial pollution, and many other sources:
Section C: Effects of lead, even at very low doses
Section D: (Summary): Lead accumulates in the body over the long-term, and is mobilized during lactation.
Section E: (Summary): Major, rapid transfers take place from a grown person's lifetime accumulation of lead to small, developing infants: In a study of 530 women at ages 65 to 87, having breastfed was still associated with an average 11% reduction in the women's lifetime accumulations of lead.
Section F: (Summary): Higher-than-average childhood exposures to lead have been found to be very measurably linked with changes in brain volume in adulthood.
Section G: (Summary): Lead is especially likely to have its harmful effects during the early postnatal period.
Section H: (Summary): Many studies have found associations between lead and mercury exposure and autism, including at levels found in a very significant minority of U.S. children.
Section I: (Summary): Many studies have shown that ADHD is associated with lead at levels found in many tens of thousands of U.S. children.
The EPA web page on lead, accessed in June of 2016, re-states its long-standing position that harmful effects of lead "may occur at blood lead levels so low as to be essentially without a threshold." 1 The CDC and the American Academy of Pediatrics concur with this position.2 According to a 2014 article in the American Journal of Public Health, "research has continually shown that no amount of lead exposure is safe," citing seven studies in support of that statement.3
Lead is recognized to be neurodevelopmentally toxic both prenatally and after birth, but (according to a document of the World Health Organization), "Early postnatal exposure appears to be more effective than exposure prenatally" in reducing IQ. (p. 175 of reference 3a) WHO also refers to a study in Mexico City, covering 436 children up to 5 years of age, which described "significant negative associations between postnatal blood lead levels and IQ, whereas prenatal PbB had no effect." (p. 166 of above-cited WHO document.)
It will be explained in this section that lead is transferred to infants in breast milk, and it is transferred in doses high enough to significantly increase risk of neurodevelopmental harm. But before that, it should be pointed out that a U.S. Food and Drug Administration study for the period 2006-2011 tested 34 samples of milk-based infant formula and found no detectable lead in any of the samples.4 In an earlier FDA test, only one sample out of 88 contained any measurable lead, and that one was at a concentration (0.007 mg/kg, = 7mcg/kg = approx. 0.7 mcg/dL4) which is considered to be acceptable in breast milk. (These results were low compared with what had been found in earlier decades, before use of lead solder in cans was discontinued.)
A 1998 study's calculations indicated that "from 36 to 80% of (an infant's) lead in blood in the first 60-90 days postpartum is contributed from breast milk."8 A 2001 study estimated the contribution of breastfeeding to three infants' blood lead levels to be 40%, 60%, and 65%, based on 58 to 66 days of breastfeeding.9 A general summary statement could be made to the effect that three months of breastfeeding would account for over half of an infant's blood lead levels, and very possibly over 70% of those levels.
The above findings are compatible with the known effect of breastfeeding in increasing infant lead levels, as reported by the U.S. CDC. A 2010 publication of the CDC includes a chart showing ten different possible breast milk lead levels across a wide range of likely concentrations (shown in Figure 2 just below); it is noteworthy that at every maternal lead level shown, including the lowest (1 mcg/dL), and even with only one month of nursing, breastfeeding is shown to always result in an increase in blood lead concentrations in the infant.10
Fig. 2 (chart from CDC document, see ref.45)
Since even the lowest maternal lead level shown by the CDC (1 mcg/d) would increase a breastfed infant's blood lead, it is worth noting the blood lead levels that are typical for tens of thousands of women, according to the latest data that appears to have been published as of 2016: the CDC states that 10.5% of women age 20 to 49 tested in New York City in 2004 had blood levels equal to or greater than 2.5 mcg/d;.10a that figure did not include the major increase that takes place in women's blood lead levels at about the time of birth (see Figure 3 and accompanying text).
The increases in infant lead levels may be small with each individual feeding of breast milk; but many small amounts can add up to significant totals, especially considering that (a) lead is known to accumulate in the body,11 and (b) according to the EPA, harmful effects of lead "may occur at blood lead levels so low as to be essentially without a threshold."12
So (picking up on a topic started earlier) breastfeeding has been found to commonly become the source of half or more of an infant's lead levels even after less than three months of breastfeeding, as found in the 1990's. In the 2000's in the U.S. and most European countries, breastfeeding would normally be
(a) of much longer duration than three months, on average (see Figures 6 and 7 below), and
(b) more likely to be exclusive than during the 1990's, when breastfeeding for 3 months was found to be the source of half or more of an infant's lead levels. (Figure 6)
So breastfeeding for three months -- before exclusive breastfeeding was widely done -- was already commonly providing over half of a typical infant's lead levels two decades ago, according to studies; and typical contemporary breastfeeding would normally be contributing much more than that. With that in mind, think about the 2016 policy statement on lead toxicity of the American Academy of Pediatrics, in which it says (in line with the EPA's position12), "it is now recognized that there is no safe level of lead exposure," and "all sources of lead exposure should be eliminated."15 (emphasis added) The U.S. ATSDR holds that same position, saying "It is important to prevent all lead exposures."79 Notice that these appear to be firm recommendations, not allowing exceptions if the doses are below a certain level or for any other reason.
It is sometimes pointed out that concentrations of lead in breast milk are much lower than in maternal blood; the apparent intended implication is that transfer of lead via breastfeeding is therefore not very significant. But that would be a false and misleading impression to convey, for reasons as follows:
-- a) lead accumulates in the body;18 numerous small doses add up. The eventual result of that "slow and steady " transfer of lead is likely to be especially significant for an infant, considering the large capacity of the source as compared with the small size of the receiving body.
-- b) Absorption of lead from the intestine has been found to be 40 times higher for children than for adults;17 a little lead goes a long way in an infant. A 2008 Chinese study found that, after adjustments, "every increase of breast milk in 1 μg/L unit resulted in the increase of blood lead of infant by 14 μg/L."82
Remember from above the CDC's estimates that breastfeeding would result in an increase in infant blood lead concentrations even at the lowest levels of maternal lead that the CDC considers it to be appropriate to report about. And bear in mind that, according to the U.S. Food and Drug Administration, contemporary infant formula in the U.S. has been found to contain essentially no lead.4
Soil and dust are sources of child exposure to lead in addition to food, after early infancy; but in the early months after birth, those are considered to be of little relevance.20 Infants would not yet be at the stage of normal hand-to-mouth activity until 6 months after birth,21 which would be past the especially vulnerable early postnatal months. (see Fig. 4.a and reference 22) Breastfeeding would therefore typically be the predominant source of lead in infants during the developmentally-sensitive early months after birth.
There is excellent evidence linking lead, including in the concentrations that are found in many tens of thousands of infants in the contemporary U.S., with greater risk of both autism (see Section H later) and ADHD (see Section I later).
To summarize some key points from above:
-- lead has been found to be highly absorbed by infants and to accumulate in the body even though it is received in very small doses;
-- U.S. infant formula of recent years has been authoritatively found to contain no detectable lead;
-- lead received via breastfeeding, especially with more than two months of nursing, is likely to constitute most of the lead in an infant, at the developmentally most vulnerable time of the child's life;
-- according to eminently authoritative sources, with no apparent disagreement, "...it is now recognized that there is no safe level of lead exposure;"
As will be described laler in this article, there is substantial evidence in numerous studies linking lead, in concentrations that are found in many tens of thousands of infants in the contemporary U.S., with greater risk of autism (see Section H) and ADHD (see Section I).
a) occupational and hobby exposures of the mothers and of household members who bring home deposited dust: exposures occur in manufacturing of ceramics, glass and plastics, and home renovation, pottery making, jewelry making, use of dyes, and many other categories;4, 23, 24 in a Chinese study, lead in breast milk was found to be almost 12 times as high in occupationally lead-exposed women as in occupationally non-exposed women.25
b) Cosmetics: Traditional eyeliners popular in many parts of the world are high in lead, and have been linked with lead poisoning;26,.27 The U.S. Food and Drug Administration had a survey done in 2010 analyzing lead contents of 400 different lipsticks purchased in retail stores, in which lead was found in all lipstick samples tested; 11 of them, mostly brand-name products, had lead contents of over 4000 parts per billion;28 for comparison, the EPA's action level for lead in drinking water is 15 parts per billion;28a bear in mind that (a) lipstick is applied (often many times a day, every day) at a location where it can easily rub off and enter the body in connection with eating and lip licking, and (b) lead is known to the U.S. ATSDR to be absorbed through the skin as well as by inhalation,29 both of which would be relevant to lipstick; also (c) lead accumulates in the body, over decades (more on this below).
c) according to the U.S. FDA, a child-bearing woman's diet contributes an average of 43% of her lead levels;30 the FDA says that "many food products would be expected to contain very small amounts" of lead; the FDA has found lead in juices, fruit, carrots, sweet potatoes, candy, and other common foods.30a A Polish study found lead present in vegetables in general, especially in potatoes, grains and cereals.30b Apparently those "very small amounts" add up over the years.
d) Other common sources are industrial and cottage industry emissions (which are especially high in developing countries),23 urban pollution (including in developed countries),31 use of traditional lead-glazed pottery, eating of non-food substances (pica -- which is common in Africa, Latin America, and U.S. immigrant communities), culturally-specific practices (such as use of lead-containing alternative remedies),32 leaded paint and leaded gasoline exhaust in the many countries in which those are still legal or only recently banned; and residence in older housing even in developed countries. Leaded fuel is still used in piston-engine aircraft, of which there are currently over 150,000 in the U.S., and over 16 million Americans live within a kilometer of airports used by such aircraft. Several hundred thousand tons of lead are emitted into the air annually by industrial processes and electric generation in the U.S., as of 2008 data.33 (According to the U.S. ATSDR, "almost all inhaled lead is absorbed into the body."34) In a town with industrial pollution, lead concentrations in breast milk were found to be 420 parts per billion, compared with 30 ppb in the control group.35
Remember from above that the FDA, in its most recent test, found no detectable lead in scores of samples of infant formula. Cows and their milk benefit from much less exposure to the many lead sources indicated above as well as from continuous clearing of toxins out of cows' bodies via regular milking.
Section C: Effects of lead, even at very low doses, are well known, but to quote the U.S. CDC for a brief summary, lead increases the risks for:"damage to the brain and nervous system, slowed growth and development, learning and behavior problems (e.g., reduced IQ, ADHD, juvenile delinquency, and criminal behavior), and hearing and speech problems."36
The American Academy of Pediatrics states, "low-level lead exposure, even at blood lead concentrations below 5 µg/dL (50 ppb), is a causal risk factor for diminished intellectual and academic abilities (and) higher rates of neurobehavioral disorders such as hyperactivity and attention deficits. No effective treatments ameliorate the permanent developmental effects of lead toxicity."
A 2015 study, summarizing results of earlier studies, states, "Even low-level exposure to lead that is prevalent in daily living has been associated with reduced intelligence, impaired attention, and behavioral problems."37 Experts have provided other authoritative evidence of adverse cognitive effects at very low levels of lead.38 In a 2006 study, major increases in ADHD were found even in children with blood levels below 2 µg/dL (2 mcg/dL, or 20 parts per billion). (see Figure 5) An earlier study showed no evidence of a harmless level of lead going down to 1 µg/dL39 According to Health Canada, "several studies have modeled a dose–response relationship that extends down to the lowest blood lead concentrations studied (1–2 μg/dL)."40 A study published in 2016 found that an average lead level of only 1.64 μg/dL was associated with more autistic behaviors.41 A 2009 study found elevated ADHD risk at lead levels in the 1-1.5 μg/dL range. Another 2016 study found lead levels as low as 1.2 μg/dL to be associated with increased ADHD.43
Given the substantial evidence of harmful effects of blood lead levels at or near 1 μg/dL, note that CDC data shows that 3% of U.S. children in 2015 have blood lead levels five times that high, or higher.44 Obviously, a much higher percentage has lead levels that are less than 5 μg/dL but still well within the hazardous range.
In the CDC chart below we can see how easily an infant's lead levels, via breastfeeding, can get into the range that has been found in the above studies to be hazardous to the developing child.
Fig. 2, repeated (chart from CDC document, see ref.45)
Consider the outcome if a mother's blood lead level were to be at least 8 μg/dL, which is a very mid-range figure in this CDC chart; it is also within the "normal" range according to data accessed in 2016 from the U.S. National Library of Medicine.46 Breastfeeding for only one month by a mother with such a lead level would lead to an increase in infant lead level of 0.98 μg/dL (see that figure in the third column above); that increase of the infant's lead level due to one month of breastfeeding, combined with the lead accumulation that the infant would have received prenatally, would quickly get the infant's lead level into the range that has been found to be linked with increased risk of neurological problems, in the studies summarized above. Similarly, if a mother's lead level was only half of the above "normal" level (fourth line on above chard), breastfeeding for only two months would put the infant's lead level into the range that has been identified as hazardous.
Considering how many infants are breastfed for six months and more, this data indicates what should be reason for serious concern regarding transfers of lead via typical breastfeeding.
Even though the above chart shows a wide range of women's blood levels, it does not show the full range at the high end. Milk of women living near a smelter in Mexico were found to have average lead concentrations of 62 μg/L.46a
It should not be surprising that a CDC publication on the subject of lead in lactating women, in its section on "Research needs," listed the following: "development of new therapeutic agents or mechanisms to remove lead from breast milk."47
There is also considerable other evidence linking lead, in concentrations that are found in many infants in the contemporary U.S., with greater risk of autism (see Section H) and also with greater risk of ADHD (see Section I).
The EPA points out that, in addition to many other adverse effects, "lead damages the development of the electrical connections in a child’s brain."49 It is significant that formation of those connections in the brain is especially rapid during the early months after birth, the same months during which exposure to breastfeeding (with its recognized contents of lead) is especially high.49a
Aside from effects on developing children, according to a leading expert, "some evidence has shown that early-life lead exposure is a risk factor for the development of late-onset Alzheimer's disease." (citing three studies in support of that).43a
With the above in mind, remember (from the beginning of Section A) the absence of lead in U.S. infant formula in the last decade or so, as found by the U.S. Food and Drug Administration. And bear in mind that breastfeeding was rare for the generation born in the mid-20th-century U.S.;48 that generation did not have the major increases in non-communicable diseases that have become commonplace among children since breastfeeding became widespread. (see Section 13b of www.pollution-effects.info)
-- Blood lead levels below 2 µg/dL have been associated in many studies with neurological impairments. (see above)
-- CDC data shows that over 3% of U.S. children in 2015 have blood lead levels much higher than that, indicating that many thousands of U.S. infants would have lead levels well into the range associated with neurological impairment.
-- Among infants breastfed for three months or more, which applies to most U.S. and European infants in the 2000's, the infants' blood lead levels will normally be mainly a result of breastfeeding. (see Figure 1 and following text)
-- It should be reasonable to assume that, largely as a result of breastfeeding, a significant percentage of U.S. infants have lead concentrations in a known hazardous range.
Section D: Lead accumulates in the body long-term15 and is mobilized during lactation:
A 2004 study cited four earlier studies as having "clearly shown that maternal bone stores of lead are mobilized to a marked degree during lactation." 47 Three other studies have also found regular increases in mothers' blood lead levels after birth,50 ,51 in line with the recognized mobilization of stores of lead from the mothers' bones during lactation.
A 1994 study by a team of five researchers (Muldoon et al.), published in the American Journal of Epidemiology, found evidence of the large proportion of a woman's long-term lead burden that is transferred to her infant via breastfeeding: In that study of 530 white women at ages 65 to 87 from Baltimore and rural Pennsylvania, carried out in 1990-1991, having breastfed was still associated with an average 11% reduction in women's lifetime accumulations of lead.52 Put another way, of the total stores of lead that a fully-grown body had accumulated over five to seven decades, loss of an average of 11% of that was associated with transfers to a woman's small, developing infants within relatively short durations. Long-term blood lead levels in the elderly women in the above study were also reduced merely in relation to having given birth, but they were reduced by only half as much as the reductions in women who had breastfed.
A 2020 study, based on data from several separate U.S. National Health and Nutrition Examination Surveys, provides additional information on this topic. It was found that, on average, women in the 35-44 age group in recent decades had blood lead levels about 40% higher than women in the 15-24 age group, and the levels increased stepwise with age in all four age groups surveyed.53 Therefore it would be expected that, other factors being equal, an older group would have higher blood lead levels than an equivalent younger group. In that same study, it was found that women who had ever breastfed had lead levels an average of 10% less than women who had never breastfed, even though the women who had ever breastfed would be expected to have been older on average than women who had never breastfed. (That is, given the reasonable assumption that having breastfed is closely linked with having ever given birth.) This provides additional evidence that a substantial part of a woman's lifetime accumulation of lead is transferred to her breastfed infant.
Some maternal lead is transferred to the fetus, but far more is transferred to a breastfeeding infant. Remember from the text next to Figure 1 that, by the third month of breastfeeding, most of an infant's lead levels would normally be a result of lactational transfer.
Calcium is mobilized from the mother's bones to provide that nutrient to breast milk, but lead (being chemically similar to calcium) is mobilized from bone stores at the same time, and becomes part of breast milk. In addition to the mothers whose data are shown here in this chart, all of the other mothers measured in this study (Manton et al.54) also showed postnatal surges in their blood lead levels, surges that would be expected to show up in their breast milk; the increases continued for a longer period in women who breastfed for longer. Two other studies showed similar results of major postnatal increases in maternal blood lead.55, 56
Shown below are findings from a U.S. study57 by a team of 11 researchers who are authors or coauthors of a total of over 800 scientific articles. They measured lead levels in young children (birth to 6.5 years) living in a poor area of a city "where there were many old, lead-contaminated houses." (Old houses are known to typically contain peeling, lead-containing paint.) No mention was made of exposure to any other factors in this neighborhood that were likely to substantially increase lead exposure, such as industrial pollution.
The research group used magnetic resonance imaging to measure the brain volumes of the participants when they were 19-24 years old. Brain loss associated with childhood lead exposure was very detectable, as shown in these brain images, and a dose-response effect was found; "the greatest brain volume loss was seen in participants with the greatest lead exposure in childhood."
The blood lead levels that led to these effects on the brain were not extraordinarily high. Average blood lead levels of the children (13.3 μg/dl) were only one-third as high as the level at which signs of lead poisoning have normally been observed,57 and also only one third as high as the maternal blood level that the CDC considers to be the maximum level at which the mother should breastfeed.58
As is obvious in the above chart, far greater effects were found in males than in females; this is significant in that disproportionately-high percentages of neurological disorders, as well as reduced academic performance, have been found in young males in recent decades. For much more on this topic, including about male-adverse effects of the toxins in contemporary human milk, see Section 7 of www.pollution-effects.info.
Average lead levels in the general population of children in the U.S. and other developed countries have declined greatly in recent decades as a result of public health measures, especially regarding leaded gasoline and paint. However, summarizing from earlier in this section: concentrated sources of lead in many mothers are continuing
-- a) in developed countries in many occupations, hobbies and polluted areas;
-- b) in developing countries, where the lead-reducing public health measures are typically not yet in place; and
-- c) in most countries, lead is typically present in food, and women very often use cosmetics and/or ethnic remedies that are high in lead, and they often engage in pica-related ingestion of lead.
Effects of all of the above would add up to hundreds of millions of mothers around the globe who are still heavily exposed to and accumulating lead. The lead reduction that has taken place on average in children past infancy in developed countries in recent decades has been very significant, mainly reflecting reduced exposure to leaded fuel exhaust and peeling lead paint during childhood; but there appears to be no data about trends in lead levels of infants, whose lead levels would be largely determined by breastfeeding (see Figure 1 and accompanying text), which has normally been increasing. (See Figures 6 and 7.) Bear in mind that infancy is a period of great vulnerability to developmental toxins. (Section 2.a at www.pollution-effects.info)
There appear to have been hardly any studies that tried to determine the period of maximum vulnerability to neurodevelopmental effects of lead during childhood. But there are good reasons to suspect an especially early postnatal vulnerability, particularly compatible with sensitivity to lead transferred via breastfeeding, including
-- the statement by the neurology expert, D.C. Bellinger, that lead disrupts certain steps in formation of the brain, "affecting the processes by which neuronal circuitry is established;" included are "dose-dependent effects... on synapse (connection) formation," as well as many other adverse effects on the brain's developmental processes;59 notice in Figure 2 at the link two paragraphs up how much formation of the brain's critical circuitry, as well as development of the brain in general, takes place in the early months after birth;
-- A 1994 review article found that elevated blood lead at about age 2 had a significant adverse effect on IQ. Only two other periods of vulnerability were considered in that study, as follows: time-of-birth/prenatal lead had no observed effect, and preschool-years lead had less effect than lead levels measured at age 2.60 So, considering that lead levels at age 2 are a result of accumulation up until that age, that study's finding of greatest sensitivity at age 2 (compared with 0 and 4) is fully compatible with adverse effects of lead ingested during the period of breastfeeding.
-- Also see Section 2.a at www.pollution-effects.info for considerable evidence of toxic effects of especially early postnatal exposures to toxins in general, including the U.S. Public Health Service reference to the “early months after birth” as a “critical period” for development of the nervous system;61 and a statement by EPA researchers that studies "have clearly demonstrated that when proliferation is actively occurring in a given region of the brain, it is vulnerable” to toxins.62 Observe in the charts below the especially rapid proliferation that takes place in the brain during the year after birth.
Section H: Links between autism and heavy metal (lead and mercury) exposure at levels often occurring in developed countries:
-- In a 2015 study in which 28 elements were measured in hair of 30 children aged 4-5 with ASD and 30 controls, lead was a standout in excess levels of contaminant elements, with excess concentrations of lead found in 78% of ASD cases and in only 16% of controls; mercury was in second place, with 43% of ASD cases having excesses versus 10% in controls.63
-- A 2013 study found significantly higher levels of all heavy metals (which include lead and mercury), ranging from 150% to 365%, in children with autism.64
-- In a 2009 study, body burdens of toxic metals were found to be significantly related to variations in the severity of autism, for each of four scales of autism severity. The metal mentioned first among those having the greatest influence was lead.65
-- A 2011 study found highly significant (p=.001) elevations of lead and mercury in both hair and nail samples from children with autism.66
-- Remember from Figure 4.a (just above) that the earliest months and first year after birth are central in the period of greatest growth and development of the brain; that is especially the case for formation of connections related to communication and sensory perception (right-hand chart), both of which areas are fundamental problems in autism. Then remember the EPA statement that "lead damages the development of the electrical connections in a child’s brain."49 And bear in mind that the earlier months after birth are the periods of maximum exposure to breastfeeding, with its recognized significant contents of lead (Section A)
-- As pointed out by the CDC, lead has been found to have adverse effects on children even at very low levels of exposure, and no threshold has been found below which exposure to lead appears to be safe. In a South Korean study published in 2016, studying 2473 children, moderately elevated blood lead concentrations at 7–8 years of age were associated with more autistic behaviors at 11–12 years of age. The blood lead concentrations of the group in which autistic behaviors were found in this study were at an average level of only 1.64 mcg/dL (micrograms per decileter).67 This figure should be compared with CDC data indicating that over 3% of U.S. children in 2014 (over 100,000 children), even after decades of reductions in lead levels, had blood lead levels at or above 5 mcg/dL.67a
-- Another 2011 study found significant elevations of lead in both hair and urine samples of children with autism, and significantly elevated mercury in urine samples.68
-- A 2005 study found significant elevations of lead and mercury in children with autism.69
-- A 2015 Egyptian study found that the mean levels of lead and mercury in hair of autistic patients were significantly higher than levels in controls.70
-- A 2012 study reported findings supporting "the historic evidence that heavy metals, especially lead play a role in the development of ASD," citing studies going back to 1976, and showing a mean value of mercury hair concentration seven times as high in children with autism as the reference range based on non-autistic children.71
-- Observe below the data from a 2017 Swedish study71a regarding ASD in individual members of pairs of twins, one twin with ASD and the other without ASD ("discordant" pairs). As seen in all three charts for the discordant pairs (following "b" below), the twins with ASD had substantially higher postnatal levels of lead.
The authors also examined correlations of their biomarkers with severity of autism a decade later. They found that lead also showed statistically significant, positive associations with scores on standard tests for ASD and autistic traits at those later ages.
When reading about all of the associations of lead with autism (above) and with ADHD (below), bear in mind that the lead levels of an American infant during the 2000's would usually be mainly determined by breastfeeding (or not breastfeeding). To read about how substantially transfers of lead via breastfeeding can contribute to an infant's lead levels, see Figure 1 and accompanying text, earlier.
According to a study published in 2016,72 "most published studies with low lead exposure levels... in the past decade have shown that lead is associated with ADHD at levels between 2 to 4 μg/d" (citing 7 studies); the authors cited another 3 studies for associations of lead with ADHD at higher lead levels. Adding what was learned from their own study, the authors noted that "our study is consistent with most of the existing literature suggesting that the association between lead and ADHD-symptoms occurs at levels ≤ 5 μg/dL." (that is, at lead levels equal to or below 5 mcg/dL) As noted above, CDC data indicates that well over 3% of U.S. children in 2015 had blood lead levels at least as high as the levels that were associated with ADHD in many studies.73
A review article published in 2016, based on 18 studies, arrived at a conclusion similar to that of the above 2016 study and also included elevated lead levels as having been found to be autism risk factors.74
A study published in 2006, by a team of five researchers who are authors or coauthors of over 1100 published studies among them, drew on data from the U.S. National Health and Nutrition Examination Survey 1999–2002. The authors concluded that "290,000 cases of ADHD among U.S. children 4–15 years of age are attributable to environmental lead exposure," if a causal relationship were to apply in this case, which they apparently considered to be likely.75
As seen in this chart from the above-described study, a significant dose-response relationship was found between childhood lead exposure and ADHD; a dose-response relationship is considered to be especially good evidence for causation. Notice that the exposure categories in this chart indicate actual concentrations of lead within the U.S. child population as determined in the 1999-2002 NHANES survey; the exposures investigated here were not unrealistically high compared with what would be expected in a typical environment.76
In a 2010 U.S. study, "background-levels of lead exposure were associated with ADHD in a clinically characterized sample, at the lowest levels of blood lead ever studied in relation to ADHD, and in both parent and teacher reports.... Lead exposure is a plausible neurobiological candidate for involvement in ADHD." The authors emphasized that the associations that ADHD was found to have with lead were "at population typical exposure levels."76 A 2009 Korean study found 28% greater risk of ADHD among children with blood lead levels typical of common background exposures (1- 1.5 mcg/dl) compared with children with BLLs below that level.77
In addition to evidence from many human studies, a substantial number of animal experiments suggest that lead has causal ADHD-related effects on neurodevelopment.76
But: Children's blood lead levels in the U.S. have been greatly declining while ADHD has been increasing. Doesn't that conflict with lead's being a cause of ADHD?
a) Nobody says that lead is the only cause of ADHD. There have also been substantial findings of associations between ADHD-related traits and exposures to PCBs, dioxins, and brominated flame retardants (see Section 3 of www.pollution-effects.info); the flame retardants have greatly increased in the environment, and the other two continue to be at substantial levels in human environments.
b) Note that all three of the ADHD-implicated toxins mentioned just above have been authoritatively found to be present in typical human milk in concentrations greatly exceeding established safe levels, while being at very low levels or absent in infant formula.78 Then note that breastfeeding, especially for longer periods, has increased dramatically in recent decades, greatly increasing transfers of those three toxins to infants. (see U.S. data above; see below for European countries)
c) The child blood lead levels for which there is evidence of a declining trend (according to data from the U.S. CDC) are all measured in children over one year old. There are excellent reasons to suspect that the lead levels during the developmentally-sensitive first year after birth have remained high or even increased. See Appendix A.
Section J: Long-term effects of lead:
In addition to the effects of lead that are observable in early life, there are also long-latency effects of developmental lead exposure. The ATSDR points out that "childhood lead poisoning can lead to health effects later in life...."79
As the author of the above, my role has not been to carry out original research, but instead it has been to read through very large amounts of scientific research that has already been completed on the subjects of environmental toxins and infant development, and then to summarize the relevant findings; my aim has been to put this information into a form that enables readers to make better-informed decisions related to these matters. The original research articles and government reports on this subject (my sources) are extremely numerous, often very lengthy, and are usually written in a form and stored in locations such that the general public is normally unable to learn from them.
My main qualification for writing these publications is ability to find and pull together large amounts of scientific evidence from authoritative sources and to condense the most significant parts into a form that is reasonably understandable to the general public and also sufficiently accurate as to be useful to interested professionals. My educational background included challenging courses in biology and chemistry in which I did very well, but at least as important has been an ability to correctly summarize in plain English large amounts of scientific material. I scored in the top one percent in standardized tests in high school, graduated cum laude from Oberlin College, and stood in the top third of my class at Harvard Business School.
There were important aspects of the business school case-study method that have been helpful in making my work more useful than much or most of what has been written on this subject, as follows: After carefully studying large amounts of printed matter on a subject, one is expected to come up with well-considered recommendations that can be defended against criticisms from all directions. The expected criticisms ingrain the habits of (a) maintaining accuracy in what one says, and (b) not making recommendations unless one can support them with good evidence and logical reasoning. Established policies receive little respect if they can’t be well supported as part of a free give-and-take of conflicting evidence and reasoning. That approach is especially relevant to the position statements on breastfeeding of the American Academy of Pediatrics and the American Academy of Family Physicians, which statements cite only evidence that has been
(a) selected, while in no way acknowledging the considerable contrary evidence,a1 and
(b) of a kind that has been authoritatively determined to be of low quality. a1a - a2c
When a brief summary of material that conflicts with their breastfeeding positions is repeatedly presented to the physicians’ associations, along with a question or two about the basis for their breastfeeding recommendations, those associations never respond. That says a great deal about how well their positions on breastfeeding can stand up to scrutiny.
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a1a) The Surgeon General’s Call to Action to Support Breastfeeding 2011, p. 33, at www.surgeongeneral.gov/library/calls/breastfeeding/calltoactiontosupportbreastfeeding.pdf
a2) Figure 2 in Guyatt et al., GRADE guidelines: 1. Introduction -- GRADE evidence profiles and summary of findings tables, Journal of Clinical Epidemiology, at http://www.jclinepi.com/article/S0895-4356(10)00330-6/pdf
a2a) Dr. Gordon Guyatt is chief editor of User’s Guides to the Medical Literature: A Manual for Evidence-based Clinical Practice, 2nd Edition (3rd is upcoming), copyright American Medical Association, published by McGraw Hill.
a2b) Writing in The Canadian Medical Association Journal, as quoted in “Do We Really Know What Makes Us Healthy?” New York Times, published: September 16, 2007 at http://www.nytimes.com/2007/09/16/magazine/16epidemiology-t.html?pagewanted=2&_r=0
a2c) In a review in the Journal of the Medical Library Association, only two guides are recommended for use by physicians in evaluating evidence in medical literature, one of which is the one edited by Guyatt et al., already referred to, and the other of which is by Dr. Sackett. (Journal of the Medical Library Association, Oct. 2002, User’s Guide to the Medical Literature: A Manual for Evidence-Based Clinical Practice, Review by Rebecca Graves, at httpi://www.ncbi.nlm.nih.gov/pmc/articles/PMC128970)
The previously-high levels in the over-1-year-old children were found to have been largely results of ingesting particles of lead-containing paint as well as long-term exposure to leaded gasoline emissions; both of those have declined greatly following public health measures of recent decades. We know that blood levels of the overall category of children over age 1 have declined, as should have been expected. (CDC web page at http://www.cdc.gov/exposurereport/pdf/metals1.pdf) But there appears to be no data for trends in lead levels of infants, which are very likely different from those of older children. Infants would not normally be at the stage of hand-to-mouth activity until 6 months after birth, so declines in paint chips and dust would not yet be relevant to their lead levels before that age;21, 20 and they would not have been breathing for long enough for exposures to atmospheric emissions to have had much effect on their lead levels. Their lead levels would have resulted mainly from transplacental transfer and lactational transfer. (See Section A about major effects of lactational transfer on infant lead levels.) Regarding those effects, there can be no doubt that breastfeeding has increased dramatically in recent decades. (see Figure 6, especially the line for breastfeeding at six months, and Figure 7) It is entirely possible that lead levels in infants, especially during the very developmentally-sensitive early months after birth, have increased substantially in recent decades. (See Section 2.d of www.pollution-effects.info about vulnerability during early months.)
3) Burns and Gerstenberger, Implications of the New Centers for Disease Control and Prevention Blood Lead Reference Value, Am J Public Health, v.104(6); Jun 2014 at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4062025/
3a) WHO Europe, Special Programme on Health and Environment: Effects of Air Pollution on Children's Health and Development, 2005, at http://www.ihealthbank.org/Portals/0/Environmental%20Health%20in%20Emergencies/e86575.pdf#page=167
7) Ettinger et al., Maternal Blood, Plasma, and Breast Milk Lead: Lactational Transfer and Contribution to Infant Exposure, Environ Health Perspect, 2014, DOI:10.1289/ehp.1307187 at http://ehp.niehs.nih.gov/1307187/
8) Gulson et al., Relationships of Lead in Breast Milk to Lead in Blood, Urine, and Diet of the
Infant and Mother, Environmental Health Perspectives * Volume 106, Number 10, October 1998, at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1533188/pdf/envhper00533-0085.pdf
23) Tong et al., Environmental lead exposure: a public health problem of global dimensions, Bull World Health Organ vol.78 n.9 Genebra Jan. 2000, at http://www.scielosp.org/scielo.php?script=sci_arttext&pid=S0042-96862000000900003
30a) FDA web page on lead at http://www.fda.gov/Food/FoodborneIllnessContaminants/Metals/ucm233520.htm
30b) Kot et al., Assessment of cadmium contamination in cereals, cereal products and potatoes. Bromat Chem Toksykol. 2009;3:537–542
34) According to the U.S. ATSDR, "Organic lead (tetramethyllead) is more likely to be absorbed through the skin than inorganic lead." ATSDR web page at http://www.atsdr.cdc.gov/csem/csem.asp?csem=7&po=6
35) WHO: Minor and Trace Elements in Breast Milk, Report of a Joint WHO/IAEA Collaborative Study, Geneva, 1989
37) Hong et al., Environmental Lead Exposure and Attention Deficit/Hyperactivity Disorder Symptom Domains in a Community Sample of South Korean School-Age Children, Environ Health Perspect 2015 Mar; 123(3): 271–276. at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4348739/
39) Schwartz, J: Low-Level Lead Exposure and Children′s IQ: A Metaanalysis and Search for a Threshold, Environmental Research, Volume 65, Issue 1, April 1994, Pages 42-55, at http://www.sciencedirect.com/science/article/pii/S0013935184710206
46a) Oskarsson et al., Exposure to toxic elements via breast milk, Analyst (journal), Issue 3, 1995,
51) Tellez-Rojo et al., Impact of Breastfeeding on the Mobilization of Lead from Bone, Am. J. Epidemiol.(2002) 155 (5): 420-428. doi: 10.1093/aje/155.5.420 at http://aje.oxfordjournals.org/content/155/5/420.full
-- The authors also refer to similar findings in Osterloh et al., Study of the effect of lactational bone loss on blood lead concentrations in humans. Env Health Perspect 1999;107:187–94.
52) Muldoon et al., Lifestyle and Sociodemographic Factors as Determinants of Blood Lead Levels in Elderly Women, Am. J. Epidemiol. I, 1994 139 (6), p. 599, Table 2, abstract at http://aje.oxfordjournals.org/content/139/6/599.abstract. The 11% reduction comes from dividing the average difference between the blood levels of women who had breastfed and the blood levels of women who had not breastfed (0.59 mcg -- see Table 2) by the 5.4 mcg average blood levels.
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