Thunderstorm
Asthma
Some people with severe hay fever think that their hay fever "turns" into asthma or will make them tight in the chest or wheeze. In fact, pollen can trigger asthma as well as hay fever symptoms.
Pasture grasses rely on the wind to distribute their pollen.
A single hectare of ryegrass, for example, will release hundreds
of kilograms of pollen per season. The concentration of pollen
will be highest nearest its source, but high-speed winds will
distribute pollen grains over many kilometres.
Not all allergen, however, is contained within intact pollen grains.
Other allergen-carrying particles (as small as 0.1 um diameter)
can carry grass and tree allergens. Unlike most intact pollen
grains (generally 2-60 um diameter), these small particles are
capable of reaching the small airways of the lung and triggering
asthma attacks.
They are thought to be derived from plant sap (such as that released from grass blades when mowing the lawn) and the decay of plant material. Some of them might even be derived from the sticking of allergen to particles of atmospheric pollution.
You would think that rain would help relieve symptoms of hay fever and asthma by "washing"pollen out of the air. Paradoxically, some people actually feel worse. So-called "thunderstorm" asthma has been described in epidemics in Melbourne, Wagga and London. An explanation has been found in recent studies of the way in which allergen is released and its effects on patients.
Some
grass allergen (like ryegrass allergen Lol pIX) is located on
the surface of starch granules within pollen grains. A single
pollen grain contains up to 700 starch granules of 0.6 to 2.5
um (small enough to reach the lower airways in the lung). When
it rains or is humid, these may be released with the osmotic shock
of exposure to moisture.
After it rains, the number of these small particles can rise 50 fold. And not everyone who gets these severe asthma attacks have had asthma before. They have normally had severe pollen hay fever and most have been found to be allergic to ryegrass. Presumably the massive load of small allergenic particles being inhaled straight into the lung trigger these attacks.
Source:: American Academy of Allergy, Asthma, and Immunology
Appropriate management of chronic "pollen asthma" (which probably has a similar mechanism) includes commencing anti-inflammatory asthma medication either preventatively or with the first "wheeze" of spring. Some patients undergoing immunotherapy for their hay fever find that their seasonal asthma improves as well.
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