Yet another dubious study has been making the rounds of mercury militia websites and discussion forums. The study is being played up and touted by certain very excitable and scientifically not-too-bright militia members and woo-meisters like Mike Adams as some sort of vindication of the scientifically discredited hypothesis that mercury in vaccines somehow causes autism.
It is, however, somewhat interesting in that their embrace of this bit of questionable research shows how desperate the mercury militia is to grasp to any bit of peer-reviewed published research that they can spin to their cause. It's also interesting in that it shows how easy it is to stretch research findings too far to justify a hypothesis, given that the study has so many limitations that its conclusions are, at best, tenuous and not particularly convincing. Before I look at the actual scientific paper itself, let's look at how it's being spun in the press. After that, I will show why this study's conclusions do not follow from its methodology. First, a press release published on Science Daily:
A newly published study of Texas school district data and industrial mercury-release data, conducted by researchers at The University of Texas Health Science Center at San Antonio, indeed shows a statistically significant link between pounds of industrial release of mercury and increased autism rates. It also shows--for the first time in scientific literature--a statistically significant association between autism risk and distance from the mercury source.
"This is not a definitive study, but just one more that furthers the association between environmental mercury and autism," said lead author Raymond F. Palmer, Ph.D., associate professor of family and community medicine at the UT Health Science Center San Antonio. The article is in the journal Health & Place.
Dr. Palmer, Stephen Blanchard, Ph.D., of Our Lady of the Lake University in San Antonio and Robert Wood of the UT Health Science Center found that community autism prevalence is reduced by 1 percent to 2 percent with each 10 miles of distance from the pollution source.
"This study was not designed to understand which individuals in the population are at risk due to mercury exposure," Dr. Palmer said. "However, it does suggest generally that there is greater autism risk closer to the polluting source."
The study should encourage further investigations designed to determine the multiple routes of mercury exposure. "The effects of persistent, low-dose exposure to mercury pollution, in addition to fish consumption, deserve attention," Dr. Palmer said. "Ultimately, we will want to know who in the general population is at greatest risk based on genetic susceptibilities such as subtle deficits in the ability to detoxify heavy metals."
The new study findings are consistent with a host of other studies that confirm higher amounts of mercury in plants, animals and humans the closer they are to the pollution source. The price on children may be the highest.
It sounds to me as though the article is just quoting whatever the scientists say without looking at the soundness of the research, an aspect of Science Daily that has driven me crazy on more than one occasion. However, that's pretty much how the study is being reported, with headlines such as Emissions, autism are linked, study says; New Study Says Autism Linked To Coal Power Plants; and Health Science Center discovers new link between autism, power plants.
But does the study actually provide good evidence for such a link? I've already said that it does not. Let's take a look at the reasons why.
The study itself is a followup to a widely-criticized study that the principal investigator, Dr. Raymond F. Palmer in the Department of Family and Community Medicine at the University of Texas Health Science Center in San Antonio published in 20061. That study purported to show the same thing but was viewed as uninterpretable for a variety of reasons, with its most glaring flaw being that it failed to control for urbanicity of the populations being studied. Particularly harsh was Thomas A. Lewandowski:
Lastly, the authors found that the most important determining factor for autism prevalence in their study was whether the child lived in an urban, suburban, or rural area. For example, residence in an urban school district resulted in a 473% higher rate of autism compared to rural districts. Similar findings have been reported by others (e.g., Deb and Prasad, 1994). The urbanization effect is nearly 8 times stronger than the effect suggested for mercury but is given relatively little discussion and is not even noted in the abstract. Since levels of many pollutants (including mercury) would be strongly correlated with urbanization/industrialization, this also leads one to question the mercury-autism association the authors report. More detail on the impact of residence would have been helpful. Was one particular urban area (e.g., Dallas, Houston, San Antonio) responsible for the effect? Did the authors explore how data for other chemicals correlated with autism incidence? Certainly a host of environmental and social variables associated with urbanization could be investigated as possible factors in autism. Alternatively, an increased tendency for diagnosis in urban localities could explain at least part of the increased incidence.
From Dr. Palmer's 2006 study, which was so full of holes that even antivaccinationists had a hard time defending it, I got the impression that Palmer was a man with an axe to grind. This new study2, clearly done to answer that major criticism, does nothing to change my mind. Indeed, when I see that Dr. Palmer approvingly cites the infamous "baby hair mercury study" (Holmes et al) in the introduction of the manuscript, I know right away where he is probably coming from, given that that paper was a load of poorly designed garbage bordering on, if not actually, pseudoscience. He also approvingly and uncritically cites the even worse Bradstreet et al study claiming to show that autistic children excrete more mercury in the urine. If you want to get an idea just how bad this study is, consider that it was published in the Journal of American Physicians and Surgeons and that it was co-authored with Mark and David Geier. (Say no more.) Where on earth were the peer-reviewers?
Whenever looking at a study, it's very useful to look at the hypothesis and then decide whether the methods are appropriate and adequate to answer the question asked in the hypothesis. Indeed, whenever I write a research paper or a grant, I almost always include a paragraph starting "we hypothesized that," followed by a statement and justification of the hypothesis being tested (or, in the case of a grant, to be tested). The closest I could find to a statement of hypothesis in this study was this:
The objective of the current study is to determine if proximity to major sources of mercury pollution is related to autism prevalence rates.
Fair enough, as far as it goes. However, embedded in the methodology are assumptions that the methodology must account for. First, consider how the study was done. Data for 39 coal-fired power plants and 56 industrial facilities in Texas were examined in 1998 and modeled to see if the distance from these plants of various school districts correlated autism rates in 2002, with this being the rationale:
...it is plausible to postulate that releases during 1998 would have exposure potential for a cohort who was in utero in 1997. If an effect was present, this would be reflected in the 2002 school district records--the age (5 years old) this cohort would be entering the system.
So the real hypothesis being studied, although not stated explicitly, appears to be that exposure to mercury in utero contributes to autism, not that infant or childhood exposure to mercury is related to autism prevalence. Thus, even if this were a good study, how this relates to antivaccinationist claims that mercury in the thimerosal preservative in vaccines causes autism is as tenuous as most antivaccinationists's grasp of science. Be that as it may, the next step is to see if the methodology tests the hypothesis. There are a number of problems. First among these is that it is quite unclear exactly what data from the Texas Education Agency were used. Apparently Dr. Palmer used some sort of special data set provided by the TEA that is not publicly available. Another really glaring error is this:
Total number of students reflects all enrolled students in the districts 2002 school year and was used as the denominator in calculating autism rates.
Later, under the Statistical Methods section, Palmer states:
District autism data in 2002 were treated as event counts and used as the outcome in a Poisson regression model predicted by pounds of environmental mercury release in 1998, distance to sources of the release, and the relevant covariates. Total number of students enrolled in each district for 2002 defined the rates for each district
Wait a minute! How could this be? I thought that the model assumed that in utero exposure to mercury in 1997 would correlate with autism prevalence in the five year old cohort in 2002. Yet it appears that autism prevalence in 2002 was calculated using the total number of students enrolled, not the number of students entering kindergarten; i.e., the number of students who were exposed to the levels of pollution in utero in 1997. If this statement is accurate, it tells us that Palmer was using autism prevalence in 2002 for all students, K through 12. Since this includes children up to age 18 and since autism is usually diagnosed by age 5, this methodology would necessarily include a large number of autistic children who would already have been diagnosed with autism in 1997, the year for which mercury emissions from power plants and industrial sources was measured. In other words, Palmer included in his dataset far more children whose autism, using his own hypothesis, could not possibly have been related to mercury emissions than he did children who might have been susceptible.
This flaw alone, pointed out by Michelle Dawson, if I have not missed something, makes the results of this study completely uninterpretable. The correct methodology would have been to compare autism prevalence in the kindergarten (or, if using loose criteria, perhaps the kindergarten through third grade cohort) with mercury emissions. Moreover, the perils of using special educational services data as a surrogate for true prevalence are well known.
There are also numerous other deficiencies in the design and methodology of the study, as one might expect. Another glaring flaw is that Palmer appears not to have done any tests to see if distance from power plants correlates with some other confounding variable other than urbanicity and wealth. True, he did try to control for urbanicity, mainly because urbanicity correlates so strongly with autism awareness and access to resources, making it not surprising at all that in his earlier study autism prevalence correlated far more strongly with urbanicity than with mercury emissions. It also correlates with population density, many other forms of pollutants such as auto exhaust, and many other potentially confounding variables. Other serious flaws that I found include:
- The method for calculating distance from power plants. Basically, Palmer took the geographic center of each school district, measured the distance from that to the nearest power plant, and then used that distance for every child in the district. Remember, this is Texas we're talking about. Some of these school districts are quite large, but Palmer's methodology averages the distance out for every child in the district.
- Moves. The authors appear to assume that no one moves in or out of the district.
- No examination of wind effects. The underlying hypothesis here appears to be that mercury carried on the wind is what correlates with autism prevalence. If that were the case, then it would be expected that the effect would be much stronger in school districts that, based on the general direction of the prevailing winds, are downwind from a power plant. Palmer didn't even consider this variable.
- No comparison to other regions. The EPA has a very nice map showing the distribution of deposition of mercury on a global basis. Texas has few "hot spots," while in the U.S. the Northeast and Midwest have many and China is one continuous hotspot. If Palmer's hypothesis is true, it would have been nice of him to include a spot check of autism prevalence rates in a state with a lot of coal burning power plants, such as West Virginia.
In the end, Palmer has only answered one of Lewandowski's criticisms of his 2006 study. The others stand:
Palmer et al. used county-level (and school district-level) TRI data for mercury as a surrogate measure of mercury exposure. The authors note in their introduction that mercury emitted into the air may be carried many miles before being deposited to soil or water. This is critical. Air modeling analyses indicate that mercury deposition that occurs in the west of the nation (including Texas) is overwhelmingly attributable to Asian or other non-US sources (Seigneur et al., 2004). Texas is a significant source of mercury emissions, but the mercury from these emissions is largely deposited hundreds to thousands of miles to the east. It is therefore highly unlikely that mercury emitted in a particular county or school district can be correlated with air mercury exposures in that locality.
TRI data also do not specify mercury species or the environmental medium to which the mercury is released. The likelihood of human exposure (and resulting toxicity) is highly influenced by these factors. For example, community exposure to inorganic mercury present in coal fly ash shipped to an off-site disposal facility will be zero. Releases to surface water bodies may also have a very different exposure potential than releases to air.
The authors also acknowledge that fish consumption is the primary source of human exposure to mercury. Fish mercury exposures in the general population are primarily associated with ocean caught fish, such as tuna or swordfish (Carrington and Bolger, 2002; Dabeka et al., 2004). Mercury levels in ocean fish are impacted by releases on a continental rather than a county-wide scale. Even for freshwater fish, which may be sources of mercury intake for a limited number of individuals, the mercury will most likely be attributable to distant sources. Local mercury releases (as described by the TRI data) should therefore not be used as a surrogate variable for actual mercury exposure. Because TRI-mercury releases on the county or school-district level are unlikely to be correlated with actual mercury exposures in the same geographic regions, it seems implausible that the observed association between mercury release rates and autism prevalence represents a real biological phenomenon.
It's entirely possible that some environmental factor, or factors, may contribute to the development of autism, either on their own or by acting with some genetic susceptibility, but if that is the case with mercury this study is thin gruel to use to support such a hypothesis. In fact, it's not even that good as a hypothesis-generating study. There's just too much potential interference from confounding variables that hasn't been accounted for.
Finally, it is important to note that Dr. Palmer may well be biased. For example, yesterday he spoke at a demonstration promoted by Safeminds and Autism United, both boosters of the "vaccines cause autism" lie. I tried to find an account that describes what he said there, but the best that I could find was this, which didn't say much. Given the poor quality of this study and his cozying up to antivaccination groups, one has to wonder. If he has an obvious source of bias, it may well explain much; if he does not I'm left with the less satisfying conclusion that he is just not a very good scientist at all. I also have to wonder about the quality of the peer review of this particular journal. After all, if I, who am not an epidemiologist, can spot the glaring flaws in this study, why couldn't the peer reviewers?
- PALMER, R., BLANCHARD, S., STEIN, Z., MANDELL, D., MILLER, C. (2006). Environmental mercury release, special education rates, and autism disorder: an ecological study of Texas. Health & Place, 12(2), 203-209. DOI: 10.1016/j.healthplace.2004.11.005
- PALMER, R., BLANCHARD, S., WOOD, R. (2008). Proximity to point sources of environmental mercury release as a predictor of autism prevalence. Health & Place DOI: 10.1016/j.healthplace.2008.02.001
It seems to me that Palmer and others start with the assumption of mercury's causal role in autism, and work backward from that assumption. How else can you explain their apparent blindness to other environmental factors equally deserving of attention. Isn't the whole point to actually *understand* the causes of autism, rather than just singling out one arbitrary substance to "blame"?
Thanks for posting on this. When I first saw the headline, I wondered whether the coverage matched the findings. Apparently not.
Did they factor in any other materials released by coal fired plants, such as radioactive particles, etc? See http://www.sciam.com/article.cfm?id=coal-ash-is-more-radioactive-than-n…. Regardless of any possible relevance to autism, coal plants just do not make good neighbors.
First Shaverism, and now this? Dang, the Atom really has lost it, hasn't he?
Thanks for writing about this.
In response to my query, Dr Palmer confirmed via email that K-12 TEA data from 2002 were used in his study (rather than data from a cohort whose likelihood of being autistic could have been affected by mercury pollution in 1998). This is not at all clear in the paper itself.
For anyone interested, I wrote about Palmer et al. (in press) and Palmer et al. (2006) on the TMoB board here http://www.quicktopic.com/27/H/vJvhV4fDnBgw7/m7697
You might also want to look at the EPA's map of mercury deposition (2001). This takes into account not only powerplants but all sources of mercury and shows the effects of wind drift.
I'm sure that Palmer et al aren't claiming that only mercury from power plants is able to cause autism, are they?
If not, then they need to explain why Texas, which has a significant deposition of mercury - especially in its more populated areas - has about the same autism prevalence (according to educational data from the USDE) as Idaho, which has much less mercury deposition.
He also needs to explain why Pennsylvania (which appears to be covered in mercury) has a much lower autism prevalence than Minnesota and Oregon. In short, a casual glance at publicly available data would show that this hypothesis is not supported.
It's sad to see a scientist chasing after data to "prove" his hypothesis, rather than looking for data to "test" it.
I talked to and heard a Science Daily editor speak on how they operate. As a daily publication they're forced to into an unflappable faith in the accuracy of published research. In other words, cogent, independent scrutiny is not an option at at Science Daily. Also, they'll shark over less prestigious journals in hopes of getting the scoop on the odd discovery. This invariably leads to increased hits and misses.
I've been wondering about this study since I first saw the headline a few days ago. Thanks for writing on it.
Well, since they haven't used thimerosol in vaccines for years, those autistic kids must be getting mercury SOMEWHERE, right?
Let me run a thought by this group, why is there a rush to blame the rise in autism to mecury? wouldnt that make more sence if it was on the decline from the time that people stopped playing with it as a toy?
All this focus on mercury - bah!- it's lead, I tell ya, it's lead.
No comparison to other regions.
My degree is in History and these two jumped out at me as potential problems even before you listed them. I lived in one place from birth to age 3, then another for 2 years, then another going into 1st grade. I should have thought that would make a difference.
Published in Health and Places. Another high quality, high-impact factor journal.
I remember reading a paper from 1998-2001 (somewhere in there, I forgot the exact year) that found a tentative link between metallothionein deficiency and autism. The hypothesis proposed at the end was something along the lines of "children with a genetic mutation causing defective or diminished quantities of metallothionein may be more susceptible to autism after vaccination because the thimerosal may be the final environmental insult to trigger the disease, but it would have happened eventually anyway."
I searched around and couldn't find the paper again. I did find this:
which seems to suggest that these guys, at least, believe it, but the information is for sale instead of posted freely, so I'm a bit doubtful. There are some other things to read elsewhere on the site, but I'm not well-versed enough in the field to accurately assess the usefulness of any of it.
Have you heard about this "metallothionein theory of autism," and if so, what do you think of it? It seems like it's not getting much public attention, which is weird since I've read at least one peer-reviewed paper suggesting that it may have merit, which is one more reliable paper than I've read about the direct thimerosal link.
What was considered "mercury" for purposes of the study? Elemental mercury, ethylmercury, methylmercury, any compound containing mercury in any form, anyone driving a Mercury automobile? As you, Prometheus and others have noted repeatedly, there's a lot of differences in the ways the body reacts to different mercury compounds. That type of breakdown as to the amounts of different mercury compounds would make a huge difference in the outcome of the statistical analyses.
As to the significance of the study, it sounds like I could substitute "10-micron or larger particulate emissions" for "mercury" -- or any other air emissions permitted by the Texas environmental regulatory board for coal fired power plants and industries -- and get exactly the same results. In other words, it doesn't appear that he controlled for other emissions.
I suspect that I could also look at household income levels (housing near coal fired power plants and industrial plants being less desirable and so having lower values) and show a statistically significant coorelation between lower household incomes and rates of autism as related to distance from coal fired power plants and industrial plants. Household income is frequently a fairly good proxy measure for a lot of health issues, and it doesn't look like he attempted to control for that.
Your link is from Pfeiffer Institute run by Bill Walsh. It's a treatment center where autistic kids are given all sorts of unproven and questionable supplements such as MT promoter which is a proprietary blend of amino acids designed to increase metallothionein.
... And there they are, STILL mistaking increases in diagnostic rates with actual prevalence. Not only that, but they're quietly ignoring the presence of adult autistics, and ...
I recall early proposals to institute a national airborne/waterborne chemical monitoring system to get background levels and trends for of all detectable toxics -- so any excursion would be noticed promptly.
What became of that?
Does anyone have a list of known substances and amounts that correlate with "urbanization" for example?
I recall studies of childhood leukemia done by estimating benzene releases according to distance between major traffic routes and homes a while back (Denver, I think?) as one example of how to look for this sort of correlation.
It must be part of the public health curriculum; pointer welcome, I'm asking dumb/naive questions I'm sure.
Excellent read and very thoughtful. Thanks.
Where on earth were the peer-reviewers?
That's a very important question, and I wonder if there will be any explanation for how poorly peer-reviewed this manuscript turned out to be.
Where on earth were the peer-reviewers?
That's the $64,000 dollar question, isn't it? Doubt we'll get an acceptable answer though.
I am an undergrad and I have a question for anyone who can answer it! This may not have much to do with autism, but it does have something to do with Mercury.
I just recently witnessed a thermometer being broken and it let out what looked to me like Mercury. On the ground were small little balls of silver which kept dividing into smaller balls when someone attamped to wipe it up with paper towels. We were told that it wasn't Mercury and that they don't allow Mercury thermometers anymore.
My question is: is there a similiar element that are in thermometers which acts exactly like Mercury?
Genisoy protein bars contain mercury. I wonder how many mcgs the school children in Texas and their mothers were exposed to via Genisoy bars? Perhaps JB Handley can tell us if the rate of autism has gone up in areas after Genisoy bars were introduced to the market.
It most likely was mercury. Mercury is still used in thermometers and in thermostats.
If it was in a lab it is very likely it was a mercury thermometer. There are some non-mercury thermometers that use Galinstan, an alloy of gallium, indium and tin.
It looks like a way to test if it is galinstan or mercury would be to see if it wets glass. If it does, then it is galinstan. If it doesn't, then it is mercury.
Some localities have banned the sale of mercury containing thermometers, so it could be either. The last time I checked (this was a few years ago), my local Home Depot was still selling mercury containing thermostats.
If it was a mercury thermometer, it needs to be cleaned up properly, which involves spreading elemental sulfur around and leaving it for a few days to sequester any little bits that can't be gathered. Then everything needs to be disposed of as mercury containing waste (i.e. no landfill, no incineration).
Thanks daedalus2u. I learned something new.
What is it with the plethora of 500 errors lately?
It happens all the time... read the Updated "Having problems commenting?" link below in blue.
Just seems to be worse than usual. Of coarse I've got not solid data to base that on but hey, I try to avoid having data drive my rants.
I am surprised to learn that Science Daily has editors. I thought they were just repackaging university press releases.
Anyhow, even from SD's cursory summary, it was obvious that the study assumed pollution settles orthogonally from it point source. I don't suppose anybody who ever sat around a camp fire fails to understand that it doesn't.
Anybody else suspect that Palmer isn't telling the world that he has an autistic kid, thinks he's mercury toxic, and is chelating him?
How about the Faroe Islands, where there was an excellent study of mercury levels in children at birth, measured via cord blood. Briefly, there was a 996 consecutive birth cohort that was measured at birth for mercury in cord blood. There were 747 children with cord blood mercury levels above 65 nM/L and 249 with levels above 201 nM/L. In the 1404 children born during that 2 year period including the 996 tested cohort, there were 5 cases of ASDs, 2 of autism and 3 of Asperger's.
In the infamous DeSoto et al reanalysis of the Ip et al paper, there were 2 children with blood mercury levels above 60 nM/L (one control, one with autism). They were rejected as "outliers".
So if 65 nM/L mercury in cord blood doesn't cause autism, what basis is there for thinking that much smaller amounts will cause it?
Yeah, Science Daily is basically just a press release clearing house. They have a small amount of editorial oversight, but as far as a news source, they should be considered a PR aggregator. I would never say they could have a "scoop." They're a fine source of information, so long as you know what you're getting.
When Palmer et al. (2006) came out, I don't recall the mercury militia getting so excited, even though at the time the claimed results were more impressive. I think at the time they didn't see the study as a positive, since is was about a competing hypothesis. I guess these days they are happy to take anything that remotely supports their position.
BTW, I posted my own analysis over at LB/RB.
Does he have an axe to grind? Well, the effect of urbanicity is fully 10x the supposed mercury effect.
Any mention of that in the press releases? Any, "living in cities results in a 30% higher autism rate than living in the country?"
Is this stressed (or really given any mention) in the conclusions?
Joseph's look at special ed data and mercury is very interesting at leftbrainrightbrain.co.uk
Yeah, it's personal for Palmer. He's got a conflict of interest and he needs to address it. I'm confused as to why he has not done this. The conflict may be so emotional for him that his mind has long been made-up and he's simply justifying a certain treatment plan that's being followed. This information needs to disclosed.