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SAN FRANCISCO -- Ordinary adolescents showed higher levels of a biomarker for airway inflammation in the days following rain or snow, researchers reported here.

For every 2 mm/day of precipitation averaged over the previous week, Massachusetts teens showed a 3.64% increase (95% CI 0.72-6.65) in fractional exhaled nitric oxide (FeNO), according to a group led by Nicholas Nassikas, MD, of Beth Israel Deaconess Medical Center in Boston.

The association was much stronger for the subgroup of adolescents with asthma -- 13% of the 972 teens participating in the analysis, the authors noted in a poster presented at the American Thoracic Society annual meeting. They faced an 8.55% increase in FeNO (95% CI 1.22-16.40) for each 2-mm/day increment in precipitation over the previous week.

Whether it's the precipitation itself versus other phenomena associated with it -- growth of molds or increases in pollen, to name two -- that trigger the rise in FeNO isn't clear. However, this is not the first study to link precipitation with respiratory illness. , for example, found increases in asthma hospitalizations following summer storms. Authors of that study speculated that humidity itself could be a factor, in addition to the potential for increased airborne allergens that may accompany rain events.

However, the new study of Massachusetts youth found the association held even for relatively minor amounts of rain. A heavy thunderstorm can easily drop 25-50 mm of rain, but increases in FeNO were seen at daily averages of less than 5 mm. The researchers did rule out two common pollutants known to be airway irritants -- ground level ozone and small particulate matter -- as playing a role in the relationship; adjusting for these variables made no difference.

Nassikas and colleagues analyzed data from , a wide-ranging longitudinal study centered in the Boston area that began in 1999, enrolling pregnant women with the original intention of exploring relationships between maternal diets and outcomes in offspring. It has since expanded to examine other research questions, including studies of participants' children who are now adolescents and young adults. Asthma is a particular focus.

The 972 adolescents included in the current study were about 13 years old on average and half were female. They were tested for FeNO, forced expiratory volume in 1 second (FEV₁), and forced vital capacity (FVC) at their regular visits. Mean FeNO level in their breath was 25.9 ppb (SD 26.9). Their places of residence were known and the researchers linked them to weather data over the previous week with a resolution of 800 meters.

An additive model suggested an inverted U-shaped relationship between recent average precipitation and FeNO, with the maximum effect at about 12 mm. With greater precipitation, the relationship flattened and began to decline. (Also, the 95% confidence intervals became extremely wide because Massachusetts seldom averages more than 10 mm over a given week. The median was 1.96 mm in the study.)

Importantly, though, Nassikas and colleagues found no impact on more direct measures of lung function, i.e., FEV₁ and FVC. Data on asthma exacerbations or other acute events were not included in this study.

Precipitation is not a readily modifiable risk factor, at least for individuals and localities. But the investigators noted that their study adds to the evidence that climate change, and the alterations in precipitation patterns that come with it, has health consequences for which society should prepare.

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