Thunderstorm Asthma

By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
AAAC Collection Station—Birmingham, Alabama

You perhaps have read about thunderstorm asthma or thunderstorm fever and the significant impact that it has had on some individuals. This phenomenon was first identified in the 1980’s, but is one that is still far from being fully understood. The term thunderstorm asthma describes an “observed increase in acute bronchospasm cases following the occurrence of thunderstorms in the local vicinity.” (1G. Dabrera et al.) However, there is no standard definition of a thunderstorm asthma episode used by the medical community. Certainly, you shouldn’t feel like you are an ill-informed individual if this is a new phenomenon being described to you. These events are relatively rare and for the most part have been reported on in parts of Europe and Australia, but not in the U.S. Consider the following listing of reported incidents of thunderstorm asthma:

  • 6 July 1983–7 July 1983: Birmingham, England
  • November 1987: Melbourne, Australia
  • 29 November 1989–30 November 1989: Melbourne, Australia
  • 24 July 1994–25 July 1994: London, England
  • 30 October 1997: Wagga Wagga, Australia
  • 4 June 2004: Naples, Italy
  • 2010: Melbourne, Australia
  • 2 November 2013: Ahvaz, Iran
  • 21 November 2016: Melbourne, Australia
Thunderstorm Fever

Parietaria pollen bursting from osmotic shock. Release of cytoplasmic contents containing allergens.(From article by G. D’Amato, G. LIccardi, and G. Frenguelli, Thunderstorm-asthma and pollen allergy, in Allergy 2007:62:11-16.)

Things known to be required to trigger an event of thunderstorm asthma include high airborne pollen counts and thunderstorm winds that are strong but not damaging. Pollen grains are picked up by the thunderstorm and these grains and their allergenic fragments are distributed in large numbers on susceptible individuals.

Since we don’t seem to hear about thunderstorm asthma in the Southeastern U.S., does it suggest that we don’t have thunderstorms or asthma problems here? Of course not, that is an absurd question since we have all experienced thunderstorms, from mild to severe types; and we all know people who suffer from asthma. What researchers have found is that thunderstorm asthma is an uncommon event. Most thunderstorms, even those coincident with high levels of aeroallergens, are not followed by asthma epidemics.

Aeroallergens such as pollen and mold spores are present in varying amounts and types in the aerospora based upon season and location. For example, we rarely find elevated levels of Parietaria pollen in our collections here in Birmingham. However, in parts of the Mediterranean region the level of this pollen can sometimes be quite high. In general, pollen grains are large enough (>10 µm) that they are trapped in the nose or upper respiratory tract, never reaching the bronchi or getting into the lungs. They may trigger allergic reactions such as runny nose, itchy eyes, and congestion, but rarely do pollen grains reach the bronchial regions to trigger bronchial asthma attacks. This is also true for many if not most, of the fungal spores that are allergenic; that is, they are large enough that they are trapped in the upper respiratory system before they can reach the lower bronchial regions. However, pollen grains and fungal spores can trigger asthmatic attacks if they are broken up into smaller particles which are allergenic and which are able to reach the lower bronchial areas.

What causes pollen grains to break into fragments and release allergenic particles? Osmotic shock is the probable cause of pollen grains releasing their cytoplasmic contents and creating so called paucimicronic particles, small granules in the size range of 1-5 µm. (See photograph of rupturing Parietaria pollen grain below.) Osmotic shock can occur when pollen grains are exposed to high moisture levels (for example, rainfall or high humidity levels). Water is absorbed by the pollen grains leading to a rupture of the exine and release of the internal contents of the pollen grains. As an example, rye grass pollen grains contain a large number of starch granules (perhaps 700 or so) that if released from the pollen grain are small enough to reach the lower bronchial regions. These starch granules are coated with allergens that can then trigger an asthmatic episode.

Pollen grains make up only a small proportion of the particles in the aerospora, but many are known to function as allergens and cause allergic respiratory responses in susceptible individuals. Thus, the concentration of pollen grains in the aerospora influences the degree of symptoms observed in asthmatic attacks, but there is a complex association between the concentration of aeroallergens, airway inflammation, and clinical symptoms. Air pollution for example, may contribute to asthmatic activity, and the release of pollen allergens is modified by exposure to various environmental factors such as pollution and moisture levels.

The purpose of writing this blog is simply to provide some background on what thunderstorm asthma is and what factors contribute to its occurrence. Enough evidence does exist to suggest a causal link between asthma attacks and thunderstorms. If you or someone you love suffers from asthma, then your safest bet is to stay indoors with the windows closed during times of thunderstorm activity. Don’t wait until the storm is directly over you to go indoors, since there is a close temporal association between the start of the storm and onset of asthmatic attacks. Taking quick action will reduce your likelihood of becoming exposed to high levels of allergens that are able to reach the lower bronchial regions to trigger an asthma attack.

Enjoy the wonderful outdoors but be alert and stay healthy!

1G. Dabrera, V. Murray, J. Emberlin, J. G. Ayres, C. Collier, Y. Clewlow, and P. Sachon.
Thunderstorm asthma: an overview of the evidence base and implications for public health advice. Q J Med 2013; 106:207–217.