Asthma in Infants Linked to High Pollen Levels in Pregnancy

Steven Fox

January 10, 2013

Maternal exposure to high levels of pollen during late pregnancy may be associated with an increased risk for infants being hospitalized for asthma during their first year of life, according to results from a register-based cohort study of more than 110,000 Swedish infants born between 1988 and 1995.

The study was led by Adrian Lowe, MPH, PhD, from Umeå University in Sweden and the Murdoch Children's Research Institute and the University of Melbourne in Australia, and was published online November 7, 2012, in Allergy, Asthma, and Immunology.

The authors point out that a number of prior studies have identified that pregnancy or birth during periods of increased pollen elevates the risk for allergic sensitization and disease. However, they add, "In these studies season of birth has been used a marker of pollen exposure, rather than actual measurement of concentrations of pollen."

To find out more about that link, these investigators used a variety of public registers in Stockholm, Sweden, including health records of the children and historical records tracking pollen counts, to assess the children's' pollen exposure, both in utero and after birth.

"Information from the birth and inpatient registries were linked using the Swedish identification number (unique ten digit number assigned at birth or immigration), while daily pollen and pollution levels during pregnancy and infancy were estimated based on each child's date of conception and date of birth," the authors write. The researchers then paired those data with records of hospital admissions for asthma and lower respiratory tract infections.

They report that of the cohort of 110,381 children, 940 had been hospitalized for asthma by the time they had attained 12 months of age. Historical data showed that pollen levels varied markedly from season to season and from year to year.

Even so, they note, "Exposure to high levels of pollen in the last 12 weeks of pregnancy was associated with an increased risk of asthma hospitalization ([adjusted odds ratio (aOR)] = 1.35, 95% [confidence interval (CI)] = 1.07-1.71 for highest quartile versus remaining infants)." Adjustments were made for year of birth, maternal smoking, newborn sex, gestational age, and lower respiratory tract infections.

An especially intriguing finding of the study, the authors say, was that among some children, exposure to high levels of pollen during the first 3 months of life was associated with a reduced risk for hospitalization for asthma (aOR, 0.76; 95% CI, 0.59 - 0.98), but that was true only for children whose mothers were heavy smokers.

Although the investigators say it is unclear why heavy pollen exposure would seemingly have a protective effect on infants exposed to secondhand smoke, they speculate that "birth during high pollen periods would increase the likelihood of pleasant weather, allowing the mother and newborn to spend substantial periods of time outdoors, and thus reducing the degree of passive smoke exposure in these children." The authors emphasize, however, that future research is needed to confirm this finding.

As to the overall results of the research, the authors conclude, "If replicated, these findings may lead to mechanistic studies that help elucidate the pathogenesis of late pregnancy pollen exposure on respiratory outcomes, which could have therapeutic implications."

This project was supported by the Swedish Research Council, through the Umeå SIMSAM node, Umeå University, which provided financial support to Dr. Lowe and a coauthor to undertake this project. In addition, Dr. Lowe was supported by the Australian National Health and Medical Research Council. One coauthor and data collection were supported by a grant from the Centre for Environmental Research in Ume&aring. The authors have disclosed no relevant financial relationships.

Allergy Asthma Clin Immunol. Published online November 7, 2012. Full text