A Perfect Example of how NOT to do a Study - Part Two
The dissection of the Results section will require us to dissect a few parts of the Methods section, as the two are interconnected. I will try to make it clear when I switch areas.
Despite a "...first-order hypothesis of heavy metal toxicity in autism.", the hair mercury concentrations were as follows (parts per million [ppm] +/- standard deviation):
Autistic children: 0.47 ppm (+/- 0.28)
Controls: 3.63 ppm (+/- 3.56)
We will get to conclusions later, but I know what I thought when I read those numbers.
The numbers were different enough that, despite having standard deviations almost as large as the values, there was a very strong statistical significance (p<0.000004).From the Methods section, we know that they were using a two-tailed test and these standard deviations make a one-tailed test more appropriate (for a good introduction, see here), but let's just pass over that for now.
The authors further break down the autistic group into three sub-groups - "mild", "moderate" and "severe" - and show that the hair mercury levels are lowest in the "severe" group (0.21 ppm +/- 0.18), higher in the "moderate" group (0.46 ppm +/- 0.19) and highest in the "mild" group (0.71 ppm +/- 0.30).
A number of potential mercury "exposures" are listed, with data given only for injections of Rho D immunoglubulin, some of which contains thimerosal, a mercury-containing preservative. Of the rest of the "exposure data" collected, the authors only give the correlation between the number of maternal "amalgams" [amalgam dental restorations, also known as "fillings"] and hair mercury level. This is presented in a graph which is not particularly helpful. The other "exposure data" (vaccinations and fish intake) are not displayed.
The most important part of this section is the formula, derived by "multiple regression", that correlates three of the four exposure "variables" to hair mercury level - but only in the control group. You'll note that exposure to Rho D immunoglobulin, even though its association with the autistic subjects was highly significant and much discussed, is not part of the formula. The resulting equation is:
Hair mercury level = 5.60 + 0.04( amalgam volume ) + 1.15( fish consumption ) + 0.03( vaccinations )
Since the authors make extensive use of the results of this equation, it is worthwhile to take some time to examine it closely.
From the Methods section, "amalgam volume" was calculated from the number of amalgam dental restorations (no data about actual size was collected) and was simply the square of the number of amalgam restorations. In this way, two small restorations on the sides of the teeth were calculated to release four times the amount of mercury that one large restoration on the chewing surface - a conclusion not supported by either common sense or experimental data.
The variable "fish consumption" is presumably an integer derived from the "none, little, moderate, heavy" scale and the variable "vaccinations" is presumably the number of vaccinations received prior to the haircut.
I must admit to a certain degree of confusion about the formula, since it would appear that the lowest hair mercury level would be 5.60 ppm, assuming that all exposures were zero. I could find no subsequent correction, so this is the only formula we have.
I note that the constant in the equation (5.60) is in parentheses in the article. Mr. Blaxill has (in various Internet postings) subsequently claimed to have done the actual writing of the article and since his education is in business, perhaps he was using the accounting practice of placing negative numbers in parentheses. This would allow for hair mercury levels below 5.60 ppm, but would mean that zero exposure would lead to a negative hair mercury level - a clearly nonsensical result. Either way, the formula is suspect - at best!
This is the smallest section, but it has the biggest bang! After leading the reader down the path of mercury toxicity and the consequences of poor mercury excretion (and without showing any studies or supporting data that autistic children excrete mercury poorly), the authors veer sharply away from what their data can support:
"Despite hair levels suggesting low exposure, these infants had measured exposures at least equal to a control population, suggesting that control infants were able to eliminate mercury more effectively." [my emphasis]
Earlier, I asked you to remember that the mercury exposures were calculated and not measured and I believe I have shown that the calculations are also fairly questionable. Now, in the conclusion, the authors seem to have forgotten that they never measured mercury exposure. How amazing! Even more amazingly, the reviewers and editors failed to catch this BIG mistake!
Even more amazing is that the authors, the reviewers and the editors failed to notice two other points:
 There are no data or citations provided to support the implied assertion that mercury is actively excreted in the hair.
 There are no data citations provided to supports the assertion that mercury elimination in the hair is a significant means of clearing mercury in humans - especially infant humans, who have little hair relative to their body weight.
The reason for  is that there are abundant data and studies showing that mercury enters the hair - in all studied species - passively from the blood, attracted by the sulfur-containing amino acids in hair. The only way to "impair excretion" is to cut off the blood supply to the hair - which causes it to fall out.
The reason for  is that humans, and especially human infants, have a very small mass of hair relative to their body weight - a much lower relative amount than many of the other species studied (e.g. rodents and primates). For this reason alone, excretion in the hair cannot be a major route of mercury excretion in humans.
The authors imply, in the introduction, that one of the reasons they began this study was that some autistic children had presented to Dr. Holmes' practice with low hair mercury levels. Finding that all 94 autistic subjects in this study had low hair mercury levels, they then proceded to a conclusion that was not supported by their data.
Given that the only thing they measured in this study was hair mercury level, the only thing that their data support is that autistic children have low hair mercury levels. Since mercury moves passively from the blood to the hair, the explanations for this finding are:
 These autistic children had low mercury exposure, despite what historical data collected a median of 5 1/2 years after the fact may suggest.
 The low hair mercury level is the result of a laboratory or specimen collection error that was sytematically applied to only the autistic subjects. There is not enough information in the paper to address how this might have happened, but it is my personal favorite.
 Something about autism changes the amino acid composition of hair in such a way that it has less affinity for mercury.
You will note that none of these hypotheses requires the reversal of decades of previous study results or the discovery of currently unknown biochemical pathways. They also don't support the hypothesis that mercury causes autism (and they don't refute it), which is probably why they were rejected by the authors of this silly study.