The Australasian Society of Clinical Immunology and Allergy

It seems reasonable to think that rain would relieve hayfever by "washing"pollen out of the air. Wrong! Some people actually get worse! Epidemics of "thunderstorm" asthma have been described in Melbourne, Wagga and London. Here's why.

Final Version 8 February 2001

Around 1 in 4 people with hay fever get asthma too

Some people with severe hay fever think that their hay fever "turns" into asthma or will make them tight in the chest or wheeze. Wrong! In fact, pollen can trigger asthma as well as hay fever symptoms.

Grass pollens can be wind-blown for long distances

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 long distances.

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 smaller particles are capable of reaching the small airways of the lung and triggering asthma attacks.

These particles may come from fragments of pollen grains, from plant sap (such as that released from grass blades when mowing the lawn) or the decay of plant material. Some of them might even be derived from the sticking of allergen to particles of atmospheric pollution.

Thunderstorms and weather changes can trigger asthma attacks

It seems reasonable to think that rain could 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 on how allergen can be released into the atmosphere.

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, pollen grains can absorb moisture and burst, releasing hundreds of small allergenic particles that can penetrate deep into the small airways of the lung.

Not everyone who gets "thunderstorm asthma" has had it 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.

Pollen asthma can be treated effectively

Appropriate management of chronic "pollen asthma" (which probably has a similar mechanism to "thunderstorm asthma") includes commencing anti-inflammatory asthma medication. This can be given preventatively or with the first "wheeze" of spring. Some patients undergoing immunotherapy injections (desensitisation) for their hayfever find that their seasonal asthma improves as well. So if you wheeze during Spring, see your doctor for appropriate advice.

To contact you local Asthma Foundation, telephone 1800 645 130.

It is important to note that information contained in this bulletin is not intended to
replace professional medical advice. Any questions regarding a medical
diagnosis or treatment should be directed to a medical practitioner.

References

1. Prev Med 1997 Sep-Oct;26(5 Pt 1):711-6. Morbidity on Kauai before and after Hurricane Iniki. Hendrickson LA, Vogt RL, Goebert D, Pon E.
2. Clin Exp Allergy 1997 Jul;27(7):725-36 Thunderstorm-related asthma--the epidemic of 24/25 June 1994.Venables KM, Allitt U, Collier CG, Emberlin J, Greig JB, Hardaker PJ, Highham, JH, Laing-Morton T, Maynard RL, Murray V, Strachan D, Tee RD.
3. J Epidemiol Community Health 1997 Jun;51(3):233-8. Published erratum appears in J Epidemiol Community Health 1998 May;52(5):340. Asthma and thunderstorms: description of an epidemic in general practice in Britain using data from a doctors' deputising service in the UK. Higham J, Venables K, Kupek E, Bajekal M
4. BMJ 1996 Mar 9;312(7031):604-7. Thunderstorm associated asthma: a detailed analysis of environmental factors. Celenza A, Fothergill J, Kupek E, Shaw RJ
5. Med J Aust 1992 Jun 15;156(12):834-7. Two consecutive thunderstorm associated epidemics of asthma in the city of Melbourne. The possible role of rye grass pollen. Bellomo R, Gigliotti P, Treloar A, Holmes P, Suphioglu C, Singh MB, Knox B