Monday November 24 2008
The study looked at women who had been exposed to certain chemicals
The Sun has reported that women who use hairspray in early pregnancy “more than double the risk of giving birth to males with deformed privates”. The newspaper said that the increase in risk was only seen in women who are heavily exposed to hairspray, such as hairdressers.
The study behind the article looked at the risk of hypospadias, a genital birth defect, in the sons of mothers exposed to certain chemicals. The study’s design and the way that it was carried out mean that it cannot prove that hairspray causes birth defects. Further research is needed.
Where did the story come from?
Dr Gillian Ormond and colleagues from the University College Cork in Ireland, the Imperial College London, UK, the Center for Research in Environmental Epidemiology, Barcelona, Spain and Phrisk Ltd, London, UK, carried out the research. Their work was funded by the UK Health and Safety Executive; the Department of Health; the Department of the Environment, Transport and the Regions and the European Chemical Industry Council. The study was published online in the peer-reviewed medical journal Environmental Health Perspectives.
What kind of scientific study was this?
The researchers say that hypospadias is one of the most common genital birth defects in baby boys. This is a condition where the urinary opening is shifted beneath the penis. It currently affects about one in 250 boys born in the UK.
In this case-control study, the researchers investigated the risk factors associated with hypospadias. They were particularly interested in the occupational exposure of mothers to chemicals that disrupt the endocrine system, the use of folate supplements, and vegetarianism.
In case-control studies, the characteristics (including exposure history) of cases (children with the defect) are compared with the characteristics of ‘controls’ (non-affected children). The researchers included 471 cases of hypospadias that had been referred to surgeons and compared them with a random selection of 490 children without the birth defect. All children were born between January 1 1997 and September 30 1998 in the south-east of England. Controls were matched for cases for the region they were born in and when they were born.
The mothers of all case and control children were interviewed over the telephone between September 2000 and March 2003. They were asked about parental age, ethnicity, education, income, family history of disease, pregnancy history, maternal occupation, vegetarianism and other dietary questions including vitamin use, use of folate supplements, use of alcohol during pregnancy, smoking, demographics and domestic and environmental exposure to chemicals.
The researchers rated the women’s exposure to various chemicals during their first three months of pregnancy in different ways. For hairspray exposure, they asked the women to self-report their exposure during their first trimester.
For phthalates and other endocrine disrupting chemicals, the women were put into one of seven exposure categories dependent on their job title. This was chosen from a list of 348 possible job titles that had been assessed by a panel of occupational hygienists for the likelihood of exposure to different chemicals including pesticides, phthalates, polychlorinated organic compounds, alkylphenolic compounds, bi-phenolic compounds and heavy metals.
Based on this list, women were categorised as either ‘exposed’ or ‘unexposed’ to these chemicals. The link between the mother’s exposure status, their vegetarianism and their use of folate supplements and whether or not their baby had this particular birth defect was quantified.
What were the results of the study?
After taking into account other factors that might be associated with the risk of birth abnormalities (e.g. income, level of education, maternal age, gestational age), the researchers found that exposure to hairspray during pregnancy was linked to an increased likelihood of birth defects (OR 2.39, 95% CI 1.40 to 4.17).
As a group, there was no statistically significant increase in risk for hairdressers. Exposure to phthalates increased the risk too, but this was barely statistically significant (OR 3.12, 95% CI 1.04 to 11.46). Folate acid supplementation reduced the risk by 36% (OR 0.64, 95% CI 0.44 to 0.93). There was no link between vegetarianism and hypospadias.
What interpretations did the researchers draw from these results?
The researchers conclude that this is the first study to report that exposure to phthalates and hairsprays increase risk of hypospadias while folate supplementation protects against risk. They say that their findings with respect to folate use may have “important implications for public health and prevention”.
What does the NHS Knowledge Service make of this study?
There are several points to bear in mind that affect the interpretation of these results. Firstly, case-control studies by their nature have some biases including ‘recall bias’. Women were asked to recall their exposure to various chemicals (at work and at home) and other details about their diet and supplementation during their pregnancy. The study included children born between 1997 and 1998, but the women were interviewed between 2000 and 2003. This means that some women may have been recalling details from more than six years in the past.
There are probably inaccuracies associated with this and mothers of cases may have systematically remembered their exposures differently to controls, given their child’s problems. Although the researchers “believe that it is unlikely to explain their findings”, recall bias such as this affects results and in general is a limitation of the case-control study design.
In this particular study, exposure to some chemicals was determined by categorising women based on a list of job titles that had been risk assessed by occupational hygienists. After taking into account (adjusting for) other factors, the researchers found exposure to phthalates seemed to increase risk. However, this result should be interpreted cautiously given its borderline significance and wide confidence interval (which means it wasn’t a very precise estimate). It is possible that some women were misclassified through this approach.
There were only small numbers in the exposure groups, for example, 14 cases and four controls in the group determined by the occupational hygienists to be exposed to phthalates. Given these small numbers and also the fact that the researchers did not account for the multiple comparisons they were making in their analysis, it is not possible to be sure the results here did not arise purely by chance.
The study found that women with an occupational exposure to hairspray had an increased risk of having a baby boy with hypospadias. However, after taking into account other important factors the increase in risk for hairdressers was not statistically significant. Given the weaknesses of the study design and the way that it was carried out, its results are difficult to interpret. Case-control studies cannot prove causation, so this study cannot prove that exposure to hairspray or to phthalates was the cause of this birth defect. It also cannot prove definitively that taking folate supplements protected women against hypospadias.
Pregnant women are advised to take folate supplements as this has been shown to reduce the risk of other birth defects, notably, spina bifida. In terms of occupational chemical exposures, these results should be seen as hypothesis generating and may lead to further study.
Further research will need to provide firmer evidence on the effects of exposures to substances such as hairspray. A study such as this, which has design limitations and was not in a position to investigate the ingredients of hairsprays, does not provide proof that it is harmful in the first trimester.
Sir Muir Gray adds...
In pregnancy, avoid every chemical you can.