“Pregnant women exposed to pollen shortly before birth are more likely to have children with severe asthma” is the somewhat alarmist headline in the Daily Mail.
The news is based on a large Swedish study that aimed to investigate the relationship between exposure to pollen during pregnancy and the first few months of a child’s life, and the risk of the child requiring hospitalization for asthma in the first year of life.
The researchers found that high pollen exposure during the last trimester of a mother’s pregnancy was associated with an increased risk of the child being admitted to hospital for asthma. While a statistically significant link between pollen exposure and hospitalisation for asthma was detected, the number of babies who required this was small: just 940 out of 110,381 (0.855%).
They also found that high pollen exposure during a child’s first three months of life was associated with a reduced risk of an asthma-related hospital admission, but only in the children of mothers who smoked heavily.
Despite the author’s efforts to adjust their results for confounders, it is always possible that other factors, such as a family history of asthma, influenced the results. Additional limitations include:
- It is likely that a number of the children admitted for “asthma” did not truly have asthma (misclassification) and in fact had another form of respiratory illness. This would have overestimated the rate of admissions, and if a large proportion were infectious cases the winter seasonal variation may have led to spurious results.
- Conversely, children that did have asthma may not have had severe enough disease to require a hospital admission and instead were seen in an outpatient setting, such as a GP practice.
- Hospital admissions for children was only measured in the first year of life. Admissions for asthma at older ages, when diagnosis of asthma is more reliable, would have provided more informative results.
- The main limitation, though, is that pollen exposure and hospital admissions for asthma was not measured for individuals. These were taken from population statistics (city pollen counts and hospital admission rates), making this an ecological study, which cannot prove a cause and effect relationship.
- As both rates of admission for chest problems in children and pollen counts are seasonal, much more research is needed into immune mechanisms if pollen is to be blamed for seasonal respiratory outcomes in children.
Hopefully, the upcoming research announced in the paper will go some way to addressing some of these limitations, and provide more useful information about the possible causes and risk factors for childhood asthma.