September 30, 2004
A recent report by Bornehag et al. (Environmental Health Perspectives, July 15, 2004) purporting to show a statistical relationship between phthalate exposure and asthma, rhinitis and eczema is of poor quality, makes assumptions that are unsupported by the evidence, and in some respects are even implausible. The study is flawed and of questionable reliability.
First, the report disagrees with recent experimental evidence showing no allergic responses of the kind associated with asthma when laboratory animals are exposed to various phthalates including those in the Bornehag report.1,2.
Second, there are obvious “selection biases” in the choice of the test subjects. Some of the risk factors associated with asthma were not controlled. The study did not control for total amount of dust, an important allergen, but looked only for the amount of phthalates in the dust. It also failed to control for molds, mildew, dander, or other known allergens. Perhaps most important, the study authors first identified children with asthma, and only then went looking for environmental causes. This raises the question whether the likely source of the phthalates in the dust, probably vinyl flooring, is the cause of the children’s’ asthma, or an effect. It is common practice to replace carpeting with smooth, easy-to-clean surfaces, such as vinyl, in the homes of children suffering from asthma, in order to reduce dust. So the question is, do the children have asthma because of the vinyl on the floor, or is there vinyl on the floor because they have asthma? (The authors promise to discuss selection bias in a subsequent paper.)
Bornehag et al. found a positive association between blood levels of an immune factor and levels of exposure to one phthalate, but a slightly negative association with another. The authors attempt to explain away this awkward disparity by suggesting that the two phthalates have different effects on humans, but they point to no scientific data to support this suggestion. It may be at least equally likely that that the results obtained are pure statistical chance. Animal experiments cited above show that neither of the phthalates has any effect on the particular immune factor Bornehag discusses, even at much higher doses than those the children would have received.
A final point concerning the plausibility of the study concerns levels of phthalates found. The differences in the levels of exposure between the asthma sufferers and controls were very small. And the exposure levels for both groups were exceedingly small, and far below exposure levels that have no allergic response in laboratory animals. In all, it seems very unlikely that this report demonstrates any link between phthalates and the allergic diseases cited.
1 Butala, J., David, R., Gans, G., McKee, R., Guo, T., Peachee, V., White, K., (2004). Phthalate Treatment Does Not Influence Levels of IgE or TH2 Cytokines in B6C3F1 mice. Toxicologist (accepted for publication).
2 Butala, J., David, R., Gans, G., McKee, R., and White, K. Evaluation of respiratory sensitization potential of phthalate esters. The Toxicologist 2003; 72, 51.
Link to Bornehag article:
http://ehp.niehs.nih.gov/members/2004/7187/7187.pdf
