US researchers found that women who reported high use of cleaning products, air fresheners and insect repellents had a
higher risk of breast cancer compared with women who reported using them sparingly: however they cautioned that because the
study relied on women's recall of product use, and their beliefs about how such products might contribute to cancer development
could have influenced their recall, it should not be taken as proof of a link, but sufficient reason to research it more
You can read about how Dr Julia Brody, from the Silent Spring Institute, whose headquarters are in Newton, Massachusetts, and colleagues arrived at these findings in a report that was published online in the journal Environmental Health on 20 July.
For the study, which Brody and colleagues believe to be the first to investigate cleaning products and breast cancer, they conducted telephone interviews with 787 women in Cape Cod, Massachusetts, who were diagnosed with breast cancer between 1988 and 1995 and 721 women picked at random for comparison.
Brody told the press they found that:
"Women who reported the highest combined cleaning product use had a doubled risk of breast cancer compared to those with the lowest reported use."
She said use of air fresheners and products to control mold and mildew were linked to increased risk.
In the telephone interviews the researchers asked the women about product use, their beliefs about the causes of breast cancer, and their established and suspected breast cancer risk factors.
As well as looking for any links between product use and breast cancer risk, the researchers evaluated potential recall bias by doing what they called a "stratified analysis". They did this in two ways:
- In the first stratified analysis they looked at links between product use and breast cancer incidence arranged according to beliefs about whether chemicals and pollutants contribute to breast cancer.
- In the second they looked at links between family history and breast cancer incidence and also arranged them according to beliefs about how much family history contributes to breast cancer.
The results showed that:
- Risk of developing breast cancer was twice as high in the 25 per cent of women who reported the most combined cleaning product use compared to the 25 per cent who reported the least (they compared the uppermost "quartile" with the lowest: adjusted odds ratio OR was 2.1, with 95 per cent confidence interval CI ranging from 1.4 to 3.3).
- The result was similar for combined air freshener use (uppermost to lowest quartile comparison showed an adjusted OR = 1.9, with 95% CI ranging from 1.2 to 3.0).
- There was no such link with pesticide use.
- In the stratified analyses, the odds ratios for cleaning products linked to breast cancer risk were higher among participants who said they believed that pollutants contributed "a lot" to breast cancer, and tended towards null among participants who did not report such a belief.
- In comparison, the odds ratio for breast cancer and family history was also higher among women who believed that heredity contributed "a lot", and it was not so high among the rest (OR 2.6 versus 0.7 respectively: no overlap in the CIs).
But, they pointed out, the results also highlight the difficulties of trying to tease apart valid links from those tainted by recall bias, a common problem in studies that rely on self-reports.
"Recall bias may influence higher odds ratios for product use among participants who believed that chemicals and pollutants contribute to breast cancer," wrote the researchers.
However, they did not stop there, and raised an interesting counterpoint, arising from the results from the stratification analyses:
"Alternatively, the influence of experience on beliefs is another explanation, illustrated by the protective odds ratio for family history among women who do not believe heredity contributes 'a lot' "
Speculating on the potential influence of bias recall, Brody said that when people are diagnosed with cancer, they often think about what might have caused it: what happened in the past that may contributed to the disease?
"As a result, it may be that women with breast cancer more accurately recall their past product use or even over-estimate it," she said.
On the other hand, it could be that experience of a disease also affects our beliefs about what causes it.
"For example, women diagnosed with breast cancer are less likely to believe heredity contributes 'a lot', because most are the first in their family to get the disease," she added.
In other words, it is difficult to say which way recall bias might have influenced the results and whether such effects cancel each other out or reinforce each other.
Nonetheless, argued the researchers, given that it is biologically plausible that exposure to the sorts of chemicals contained in household cleaning products contribute to the development of breast cancer because many of them contain mammary gland carcinogens or chemicals that disrupt hormones, in the light of the overall results, they recommended it would be worth doing a prospective study on the potential links between cleaning products and breast cancer that also brings in measurements from environmental and biological media. (Prospective studies avoid recall bias because they follow groups of people over time rather than ask them to recall the past).
"Self-reported chemicals exposure, beliefs about disease causation, and risk of breast cancer in the Cape Cod Breast Cancer and Environment Study: a case-control study."
Ami R. Zota, Ann Aschengrau, Ruthann A. Rudel and Julia Green Brody.
Environmental Health Published online 20 July 2010.
Additional source: BioMed Central (via ScienceDaily).
Written by: Catharine Paddock, PhD