Wednesday March 12 2008
“Expectant mothers who use a lot of household cleaning products may increase the risk of their child developing asthma”, the Daily Mail reports. The article says that researchers claim to have found a link between wheezing and asthma in young children, and their mothers’ use of cleaning products such as bleach and air freshener during pregnancy, or shortly after birth. The study found that mothers increased the risk of their child developing persistent wheezing by the age of seven by up to 41%.
This research shows an association between the use of household chemicals during pregnancy and asthma in the child. The study has limitations, as there are many causes and triggers for asthma. These include heritability from parents and environmental factors such as smoking, animal allergies, pollen, dust mites or foods, medications, infections, emotions, stress, and cold weather. The authors have taken some of these into account in their analyses.
The study did not consider the ingredients in household chemicals in detail and expectant parents should not be too concerned by the reports. Furthermore, parents from a clean household should not think that they are responsible for their child’s asthma. Cleaning products should be used in well-ventilated rooms by everyone anyway. More research is needed to understand better the complex relationship between exposure to chemicals and asthma risk.
Where did the story come from?
J Henderson from the University of Bath and colleagues from Brunel University and the University of Aberdeen carried out the research. The study was funded by the UK Medical Research Council, Wellcome Trust, and University of Bristol. The study was published in the peer-reviewed medical journal: European Respiratory Journal.
What kind of scientific study was this?
In this cohort (group) study, the authors assessed the effects of maternal use of domestic products during pregnancy on subsequent wheezing and lung function of the child up to eight years of age.
The researchers used participants from the Avon Longitudinal Study of Parents and Children, which involved 14,541 pregnant women who were due to give birth in the Bristol area between April 1991 and December 1992. Expectant mothers completed a questionnaire on various health and lifestyle aspects, which included questions on their use of 15 chemical-based household products and how often they used them.
At six months, and 18, 30, 42, and 81 months after birth, the parents were sent questionnaires on the child’s pattern of wheezing (if any) to put them into six different categories according to the age that the wheeze began and how long it persisted for.
When the children were 7 ½ , they had skin prick tests to look at their susceptibility to six common allergens (house dust mites, cat fur, mixed grass pollen, mixed nuts, peanut and milk) and the child was classed as having an allergy (being atopic) if they had a skin reaction to cat, pollen, or dust mites. At 8 ½ years of age, the children had a lung function test to examine any degree of restrictive lung function (adjusted for age, sex and height) that would indicate asthma.
Of the 15 products in the original questionnaire, the researchers conducted further analyses on the 11 most frequently used products (including disinfectant, bleach and aerosols). A score was given to each product on how frequently it was used, and these were added together to give a total composite household chemical exposure score (CHCE). The researchers used statistical analyses to look at the relationship between this score and asthma symptoms. They took into account possible confounding factors such as maternal smoking, educational level, occupation, asthma history, number of previous children, age, environmental smoke, pets, housing conditions, and the season in which the questionnaire was taken.
What were the results of the study?
From the 14,541 pregnancies, 13,988 children survived to one year of age. Half of these children had sufficient data on both wheezing symptoms and their mothers cleaning product use during pregnancy to be included in the analysis.
By age 7 ½, the proportion of children in the six categories of wheeze were as follows:
- 56.9% children never wheezed, i.e. no wheezing at any of the five time points.
- 26.7% had early-onset transient wheeze, i.e. wheezed at 0–18 months but not at 69–81 months.
- 6.3% had intermediate-onset transient wheeze, i.e. no wheeze at 0–18 months, but wheeze at 18–42 months, and no wheeze at 69–81 months.
- 5.8% had early-onset persistent wheeze, i.e. wheeze at 0–18 and 69–81 months.
- 2.1% had intermediate-onset persistent wheeze, i.e. no wheeze at 0–18 months, but wheeze at 18–42 and 69–81 months.
- 2.3% had late-onset wheeze, i.e. onset of wheeze after 42 months and before 81 months.
The average CHCE score, on a scale of 0 to 30, was 9.4. Slightly fewer children were available to compare CHCE scores with lung function and skin prick testing. Of the children who underwent skin prick testing, there was a correlation between atopy and wheezing symptoms, with 62.4% of the intermediate-onset persistent wheeze group having atopy compared to 18% of the ‘never wheezed’ group.
The researchers found a minimally significant increase in risk of only early-onset persistent and intermediate-onset transient wheeze with increased CHCE score. When they separated the group into the children whose skin prick tests showed them to be atopic and those who weren’t, they found the greatest risk figures for early-onset persistent wheeze in non-atopic children with an increased CHCE score (the 41% figure reported by the news) and for late-onset wheeze. There was also a minimally significant increase in risk of intermediate-onset persistent wheeze in non-atopic children. The use of household cleaning products did not affect the risk of wheeze in atopic children.
The researchers found no significant relationship between cleaning product use and lung function tests.
What interpretations did the researchers draw from these results?
The researchers conclude that their results are in agreement with previous studies that suggest a link between chemical use in the home and persistent wheeze in childhood.
They also say they have shown that these associations persist for transient-wheeze for up to seven years of age, and are particularly strong in children that are not otherwise demonstrated to be atopic. They speculate that this may be due to irritant effects on the developing airways either before or after birth.
What does the NHS Knowledge Service make of this study?
There are several points to consider when interpreting the findings of this large cohort study.
- The research findings for cleaning product use and childhood asthma symptoms were collected from questionnaires and therefore there is the possibility that the participants inaccurately reported the frequency and quantity of both measures. There is no indication in the research paper that asthma cases had been confirmed by a doctor and the mothers’ memories on frequency of wheeze is not confirmation of asthma. Wheeze is very common in young children who do not go on to develop asthma, and is often associated with viral infections. It is interesting to note that no statistically significant relationship was found between chemical exposure and lung function tests, which would be a more reliable indicator of restrictive lung function.
- Environmental pollutants, which includes household chemicals, are known triggers of asthma in susceptible individuals. However, only maternal use of cleaning products during pregnancy was assessed by this study. Although the expectant mothers may have reported no use of cleaning products themselves, a partner or other household member may have been using them and therefore still exposing the mother to inhalation of chemical fumes. Additionally, exposure to cleaning products following the birth of the child was not assessed and it is possible that chemical fumes that the child themselves breathed in had an effect on their asthma symptoms, rather than those they may have been exposed to in the uterus.
- Although the researchers have considered several confounding factors, others have not been considered such as childhood viral infections, or the father’s asthma history.
- This was a large cohort study, however, half of the potential children and parents were not analysed. A large number were lost and not followed up (6,854 children), and different results may have been obtained had all possible participants been included, particularly as those with missing data may differ significantly from those included (e.g. being of a lower socio-economic group).
- From this study, it is not possible to assess the quantity of household products associated with the increase in risk or specific products.
Asthma has not just one, but many causes and triggers that may make an individual more susceptible. Parents should not be overly concerned about the normal use of cleaning products while pregnant. Rooms should be adequately ventilated during and after cleaning anyway, and people should not expose themselves to excessive inhalation of the fumes. Likewise, parents from a clean household with an asthmatic child should not think that they are responsible for their child’s condition.