The hospital admission rates for asthma and airflow patterns during thunderstorms up to 80 km away were studied for six towns in south eastern Australia between 1995 and 1998. Hourly pollen counts during one severe epidemic of asthma were also studied.

The researchers concentrated particularly on thunderstorm outflows. These are created by downdrafts of cold air within the thunderstorm that sweep up and concentrate atmospheric particles, released by rain, in a narrow band of air near ground level.

An outflow preceded a third of all asthma epidemics, overall. But during late spring and summer, when pollen counts were highest, an outflow preceded almost half of all epidemics.

The hourly pollen counts during one epidemic showed that the amounts of grass pollen grains trapped in the thunderstorm outflow were between four and 12 times higher than elsewhere. Emergency calls were registered within half an hour of an outflow being recorded.

Admission rates for asthma were not pushed beyond expected levels during thunderstorms without outflows, and a cold front of air by itself had no significant impact on admission rates.

The authors conclude that the powerful action of a thunderstorm outflow sweeps up ruptured and intact pollen grains and forces them into a band of air rising up to 1 km above ground. Asthmatics who breathe in this super concentrated mix of allergens experience an acute airways response.

(Reference: Thunderstorm outflows preceding epidemics of asthma during spring and summer. Thorax 2001; 56: 468-71.)

17-May-2001