Or maybe this is about press-release writers who don't express themselves clearly. According to "Chemical in Tap Water Linked to Food Allergies", Drugwatch 12/7/2012 (emphasis added, here and throughout):
A new study in the Annals of Allergy, Asthma and Immunology links high levels of dichlorophenols–chemicals used for chlorinating tap water–to a higher risk of food allergies. According to the study, people with higher levels of these chemicals in their urine have a greater risk of developing food allergies.
Out of the 2,211 people with high levels of dichlorophenols who participated in the study, 411 had food allergies and 1,016 had an environmental allergy, according to researchers.
Or "Pesticide Suspected in Rising Food-Allergy Cases", Voice of America 12/7/2012:
The researchers identified more than 2,500 individuals with measurable levels of dichlorophenols — chemicals found in pesticides and chlorinated water — in their urine. The study team, led by allergist and immunologist Elina Jerschow, narrowed their sample down to 2,200. Out of this group, Jerschow says, 411 of the subjects had some sort of food allergy.
Or "Dichlorophenols found in pesticides, tap water may trigger food allergies" Healio 12/7/2012:
Researchers learned that 2,548 participants (aged 6 years and older) of 10,438 people in the study had measurable dichlorophenols (greater than 0.14 mcg/L) in urine and allergen-specific serum IgE testing.
Or again, "Tap Water Chemical Dichlorophenol Linked With Food Allergies: Study", Huffington Post 12/3/2012:
The findings are based on 10,348 people who participated in the U.S. National Health and Nutrition Examination Survey between 2005 and 2006. Of those people, 2,548 had detectable levels of dichlorophenols in their urine, and of those people, 2,211 were actually used for the study.
Of those 2,211 people, 411 had food allergies, while 1,016 of them had some sort of environmental allergy, researchers found.
And in the Daily Mail, "Chlorine in tap water linked to increase in number of people developing food allergies", 12/3/2012:
In a study of 2,211 American adults with the chemical in their urine, 411 were found to have a food allergy, while 1,016 had an environmental allergy.
The study in question is Elina Jerschow et al., "Dichlorophenol-containing pesticides and allergies: results from the US National Health and Nutrition Examination Survey 2005-2006", Annals of Allergy, Asthma and Immunology 109 (2012). If you glance at that paper, you'll quickly learn that 2,548 was the total number of people whose urinary dichlorophenol level was checked, NOT the number with "detectable levels of dichlorophenols in their urine"; and that 2,211 was the total number of people with both dichlorophenol measurements (zero or otherwise) and also other covariates needed for the study, NOT "the 2,211 people with high levels of dichlorophenols".
It's true that of these 2,211 people, 411 had food allergies and 1016 had environmental allergies (according to a blood test with a significant false-positive rate) — but these are the baseline rates for all the participants in the study, and tell us nothing whatever about the effects of dichlorophenols. The actual number of study participants with high levels of dichlorophenols was 225 with one high urine dichlorophenol (> 75th percentile level of any of the six isomers), and 431 with two high urine dichlorophenols. These did actually have somewhat elevated rates of food sensitivities:
How in the world did the people who wrote those articles get these simple facts so completely wrong? Well, for a start, they probably didn't read the published study, but rather just relied on the press release, "Dichlorophenol-Containing Pesticides Linked to Food Allergies, Study Finds; Chemical Also Used to Chlorinate Tap Water". And the press release says:
Among 10,348 participants in a US National Health and Nutrition Examination Survey 2005-2006, 2,548 had dichlorophenols measured in their urine and 2,211 were included into the study. Food allergy was found in 411 of these participants, while 1,016 had an environmental allergy.
Apparently the writers of the quoted articles interpreted "had dichlorophenols measured in their urine" to mean "had measurable levels of dichlorophenols found in their urine" (though that reading makes the sentence false), rather than "had the level of dichlorophenols in their urine checked" (a reading that makes the sentence true).
I should mention that the discussion section of the published study appeals to the "worms and germs are good for you" theory as a possible explanation for the association that they found:
Studies in children have found that healthy commensal flora influence mucosal immune tolerance. These flora are thought to be responsible for the transition from the TH2-predominant profile in early life to a more balanced phenotype in nonallergic individuals. Pentachlorophenol and its metabolites have a potent antibacterial effect. Contamination with pentachlorophenol metabolite 2,4-dichlorophenol has been found in produce, such as cocoa-powder and fruit juices. Therefore, drinking of chlorinated water and ingestion of crops treated with dichlorophenol-containing pesticides may alter the microbial spectrum to which humans are naturally exposed. [...]
A daily exposure to dichlorophenols through water or food may have an ongoing effect on the bacterial diversity. There are indications that the intestinal microflora of nonallergic children differs from the microflora of allergic children. In addition, altered intestinal microbiota is associated with an increase of CD4-mediated inflammation in mice. Whether changes in intestinal microflora have a similar effect on allergic sensitization in adults is not known.