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Sherriff and colleagues report an apparent association between
mothers’ self-reported frequency of use of assorted household ‘chemicals’
during pregnancy and ‘persistent wheeze’ in their offspring. Since their
paper suggests this may help explain recent rises in asthma, the
robustness of the data and other explanations for the observed statistical
association need careful consideration.
We see several concerns, including both exposure and outcome
measures, lack of a plausible biological mechanism for the suggested in
utero effect, and the possibility of both recall and selection bias.
The definition of persistent wheeze is weak, and far removed from
criteria currently considered to define asthma . Offspring are
categorised as having ‘persistent wheeze’ if mothers’ responses indicate
one instance of wheezing in three consecutive 12 month periods.
It is curious that the association appears using frequency of use
scores which treat 11 disparate product types as equal, irrespective of
composition, and remains when each in turn was removed from the analysis.
Some product types have nothing at all in common: we can see no
significant ingredient that is common even to the majority, nor any likely
shared toxic mode of action. Contrary to the authors’ suggestion
formaldehyde is little used in the products surveyed.
Self-reported frequency of use is a very poor measure of exposure:
variations in quantity of product used, duration of use and routes of
exposure (inhalation, dermal, ingestion etc) can yield scores which run
contrary to actual exposure.
We see little justification for aggregating disparate product
exposures as ‘Total Chemical Burden’ by analogy with particulate air
pollution: the action of particulates is plausibly associated more with
size, geometry and/or absorptive capacity than with chemical composition.
This concept cannot simply be extrapolated to vapours or non-particulates,
and the term itself unfortunately suggests some absorbed, retained load:
‘sum of reported product use frequencies’ would have been more
We are not aware of any substantiated toxicological effect that is
shared by all chemicals irrespective of composition, especially given that
the notion of a ‘chemical’ is essentially subjective. All matter,
natural or synthetic, is composed of atoms and molecules. How do the
products considered differ from the many household chemicals not
considered, and chemicals that make up the natural environment to which
people are continuously exposed?
It seems unlikely that the association is driven by one product type
since it remained when each in turn was removed from the analysis.
Further, previous analyses of these questionnaires for two specific types
(aerosols and air fresheners) found no association with wheeze . Per
capita consumption of cleaning products in different European countries
also shows no correlation with the patterns of asthma .
Other than chance, how else might the observed association have
arisen? Apart from confounding by other chemicals in the built or
natural environment, certain mothers may have over-reported both chemical
use and wheezing symptoms. While ‘chemicals’ are heterogeneous in terms
of composition and toxicology, the term perhaps has some homogeneous
psychological significance for certain people. We understand the question
about frequency of use was headed “Chemicals …… in your environment”.
The authors acknowledge some selection bias: “those excluded from the
analysis due to lack of chemical use data were more likely to have wheezed
at all ages than those included, and those excluded from the analysis due
to missing symptom data had, on average, higher TCB scores than those in
the analysis”. This is potentially significant since the numbers
excluded from the study for missing data (50%) are large.
Scrutinising household products for any possible role in the rise in
asthma is important but, to be meaningful, exposures must be properly
1.Sherriff A, Farrow A, Golding A, the ALSPAC Study Team, Henderson
J. Frequent use of chemical household products is associated with
persistent wheezing in pre-school age children. Thorax 2005; 60: 45-49
2.Asher MI and Weiland SK on behalf of the ISAAC Steering Committee
(1998) The International Study of Asthma and Allergies in Childhood
(ISAAC). Clin Exp Allergy 28, Suppl 5, 52-66.
3. Farrow A, Taylor H, Northstone K, Golding J. Symptoms of Mothers
and Infants Related to Total Volatile Organic Compounds in Household
Products. Arch Environ Health. 2003 Oct;58(10):633-41.
4. Pickup J. Trends in home and consumer hygiene. In 'Are we too
clean? - a question of immunity balance'. RIPH Symposium Report. Published
as a supplement to Health & Hygiene. London: Royal Institute of Public
Health 2003: 6-7.
Dr A N Williams
Director General UKCPI