IMPORTANT The information provided is of a general nature and should not be used as a substitute for professional advice. If you think you may suffer from an allergic or other disease that requires attention, you should discuss it with your family doctor. The content of the information articles and all illustrations on this website remains the intellectual property of Dr Raymond Mullins and cannot be reproduced without written permission.

Summary
You would think that rain would relieve hay fever by "washing"pollen out of the air. Wrong! Some people actually get worse! Epidemics of "thunderstorm" asthma have been described in Melbourne, Wagga, London and Canada. Here's why.

Pollen can trigger 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.

Allergen-containing particles can be small or large
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.

Does rain make hay fever and asthma better or worse?

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, London and Canada. Instead of a few people with bad asthma, there might be dozens. In the 2016 thunderstorm asthma event, there were 3500 acute asthma presentations within a single day and 10 deaths. Not everyone who gets these severe asthma attacks have had asthma before. They have normally had severe pollen hay fever, most have been found to be allergic to ryegrass and most are not on asthma preventers. An explanation has been found in recent studies of the way in which allergen is released and its clinical effects.

Being ryegrass allergic is the major risk factor for thunderstorm asthma. 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. It is thought that intact pollen grains are swept up into developing storm clouds, fragment from exposure to condensing moisture and that the fragments are blown ahead of the storm front to ground level, triggering acute asthma in susceptible individuals.

Mold may also be a trigger

But pollen may not be the only trigger; some studies have shown that mold spores may increase at the same time, and may also trigger attacks in those allergic to some inhaled mold allergen.

Management of pollen asthma

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. Many patients undergoing immunotherapy for their hay fever find that their seasonal asthma improves as well. For acute attacks of thunderstorm asthma, follow your action plan and seek medical assistance if symptoms are worsening and failing to respond to treatment. Those prone to thunderstorm asthma may get some relief by staying inside with the doors and windows closed during this type of weather. And have a look at the National Asthma Campaign website.

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Last reviewed 29 May 2020