ENVIROMENTAL RELEASES OF MERCURY AND AUTISM
Researchers, based at the University of Texas Health Science Center set out to determine if a link can be drawn between environmental releases of mercury and the incidence of autism. The abstract reads as follows:
The association between environmentally released mercury, special education and autism rates in Texas was investigated using data from the Texas Education Department and the United States Environmental Protection Agency. A Poisson regression analysis adjusted for school district population size, economic and demographic factors was used. There was a significant increase in the rates of special education students and autism rates associated with increases in environmentally released mercury. On average, for each 1000lb of environmentally released mercury, there was a 43% increase in the rate of special education services and a 61% increase in the rate of autism. The association between environmentally released mercury and special education rates were fully mediated by increased autism rates. This ecological study suggests the need for further research regarding the association between environmentally released mercury and developmental disorders such as autism. These results have implications for policy planning and cost analysis.
The implications of this study, scheduled for future publication in Health & Place, are obvious. If environmental mercury has a casual connection to autism, then shouldn’t the same hold true for the injection of thimerosal into our children? Critics will argue that a possible link to environmental mercury discharges diminishes the impact of thimerosal by showing an alternate causal source. (The underlying causation theory of this study is that the burning of fossil fuels discharges inorganic mercury into the air, which, after being deposited onto soil or water, is converted into toxic methylmercury by chemical reactions or exposure to bacteria.) On the other hand, common sense (there I go again, resorting to common sense) dictates that if environmentally released mercury poses a danger, wouldn’t mercury injected directly into children be even more dangerous? And assuming the danger posed by mercury in general, it’s hard to argue against thimerosal being the most pervasive source of mercury exposure, at least prior to the much ballyhooed, and debateable, cut-back of thimerosal in vaccines.
I have often stated that autism is too complex a problem for us to oversimplify explanations for a cause of the recent ASD epidemic in the United States. But a hypothesis that mercury exposure, particularly from childhood vaccines, is a big piece of the puzzle makes sense to me. So when I see a study that links mercury and autism, I take note.
The Texas study is not going to be immune from criticism. I expect that we will see some commentary decrying the use of a Poisson regression analysis. I am not a statistician, and I would certainly invite any comments from those who are better versed in that discipline than I am. That being said, my understanding is that a Poisson distribution predicts probabilities of occurrence based on average rates in a discrete time period. If I understand the Texas study, the regression was used to account for demographic differences between various school districts included in the statistics. But the use of any regression analysis opens up the study to claims of bias.
Methodology aside, the real criticism we may expect is that the study can only show association and cannot show causation. That is a criticism with which the authors of the paper agree:
. . . these results indicate that the association between mercury release and school district special education rates was completely accounted for by increased rates of autism. This indicates that, in Texas, the increase in special education rates attributable to environmental mercury can be explained by increases in autism. The results of this study are consistent with our prior nation-wide study where an association between various developmental disabilities and environmentally released mercury was observed at the state level (unpublished manuscript). However, the results of this report should be interpreted with caution for a number of reasons.
First, this is an ecological study that precludes interpretation at the individual level. We have used aggregate units in this analysis to investigate differential rates of autism as a function of pounds of mercury at the county level. While we properly addressed the potentially biasing effects of clustering (school districts nested within counties) by utilizing appropriate analytic methods (e.g. multilevel-analysis), individual data are required to make a better case for the observed associations and their interpretations. Nevertheless, ecological studies of this type are often an important first step in identifying subsequent areas of investigation.
Second, a causal association between environmentally released mercury and developmental disorders cannot be determined from this cross-sectional data. Data availability permitting, future studies could investigate this association by using longitudinal data where changes in mercury levels over time may be used as a predictor of the rate of change in developmental disorders over time.
Third, we should consider that school-based administrative autism data, such as these, are only a proxy for true community prevalence. However, these autism rates are most likely biased downward.
R.F. Palmer, et al, Environmental Mercury Release, Special Education Rates, and Autism Disorder: An Ecological Study of Texas.
My own reservation with the Texas study is the same problem that any statistical analysis has. It cannot prove causation. Indeed, I would be hypocritical if I did not acknowledge the weaknesses of a statistical analysis like this one after so often criticizing the epidemiological studies relied upon by those who deny a connection between vaccines (particularly those containing thimerosal) and the triggering of ASD because the input data can be too easily manipulated -- by either design or error -- to be a reliable indicator of causation. There is no such thing as a perfectly designed analysis as long as the conclusions depend on arbitrary decisions concerning what data to use, and how to use it.
So as enticing as the Texas results are, let’s not overstate the case. The study is no more than what the authors call it: “a necessary first step in identifying plausible contributing factors of risk for developmental disabilities.” I could certainly quibble with the use of the term “first step,” as I believe the first steps have already been taken.
On the other hand, let’s not understate the case either. The Texas study is an important step toward gaining an understanding how a combination of genetics and environmental factors can trigger ASD; it’s another piece of the puzzle. It cannot be ignored; it must be followed up.