'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 hospitalisation 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.
While the evidence provided suggests that both of these associations are valid, this study provides little hard information on why either of these were detected, only theories.
The study design cannot prove that there is direct cause and effect at play (causal), as there may be other factors at work (confounders), such as the effect weather has on a mother's smoking habit.
While this is an impressive piece of research that must have involved a great deal of time and effort, somewhat frustratingly it raises more questions than it answers. The researchers have announced that they are planning further investigation into the issues raised by their study.
The study was carried out by researchers from the Occupational and Environmental Medicine Department of the University of Umeå in Sweden, and other institutions. It was funded by the Swedish Research Council, Umeå University and individual research grants.
The study was published in the peer-reviewed journal Allergy, Asthma and Clinical Immunology.
Despite the scaremongering headline that suggests "severe asthma" in individual children is directly the result of pollen exposure, the Daily Mail did show caution in the main body of the story and quoted the authors' calls for more research.
This was an ecological study investigating the relationship between seasonal exposure to pollen in the population and the seasonal rates of hospitalisation for asthma in infants under 12 months old.
Ecological studies can be useful for looking at possible associations between various lifestyle factors (such as exposure to pollen during pregnancy) and health outcomes (such as a child requiring hospitalisation due to asthma) at a population level.
They rarely allow researchers to show cause and effect, especially if both exposure and outcome are seasonal. They can usually only highlight possible associations, which then need to be followed up by further research.
The researchers explain in the introduction to the study that there is a great deal of uncertainty as to whether exposure to allergy-causing substances (allergens) such as pollen during pregnancy and early life either increases or decreases the risk of a child developing an allergic condition such as asthma.
The researchers included all vaginally-delivered babies in the greater Stockholm area from 1989 to 1996 (110,381 babies) using data obtained from the Swedish Medical Birth Registry and Inpatient Registry. Information collected from the registry included:
Based on the date of the child's estimated date of conception and birth, the researchers looked up the average pollen levels in the city for three periods:
Daily pollen levels for the time periods were obtained from the Swedish Museum of Natural History (measured at a single location in central Stockholm), while pollution levels were obtained from the City of Stockholm Environment and Health Administration.
The main outcome the researchers were interested in was any hospital admissions for asthma during the child's first year of life, using information obtained from the Swedish Inpatient Registry between 1989 and 1997.
Details on the number of admissions for lower respiratory tract illness were also used as an indication of the amount of exposure to pollens in the first three and six months of life.
The researchers analysed their results using three different statistical methods and adjusted the results for infant gender, gestational age, maternal smoking and season of birth.
They first looked at increases of pollen levels dependant on the season of birth for each of the three measured periods.
They then adjusted their results for pollution levels, and also adjusted for rates of hospitalisation in the first three months of the child's life and in the period from three to six months of life.
The researchers also assessed whether the effect of pollen exposure varied between the years the study took place by comparing the highest 25% of exposed children with the remaining children for the same year.
Of the 110,381 children, 940 (0.85%) were hospitalised due to asthma in their first year of life.
The main findings of this study were:
When looking at the highest 25% of children exposed versus the remaining children:
The researchers conclude that high levels of pollen exposure during late pregnancy were unexpectedly associated with a higher risk of hospitalisation for asthma within the first year of life.
In addition, children exposed to high levels of pollen in infancy had a reduced risk, but only in the children of mothers who were heavy smokers.
The researchers provide a number of possible reasons for the association, including:
It should be stressed that all of the above are just unproven theories.
This large study provides some evidence of an association between exposure to pollen during pregnancy and the risk of a child being admitted to hospital for asthma. Importantly, it does not provide evidence that exposure to pollens during pregnancy leads to childhood asthma.
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:
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.