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What is thunderstorm asthma?
Thunderstorm asthma is a rare dramatic onset of severe asthma in pollen allergic people during the pollen season.
How long have we known about it?
This syndrome is well-documented in the allergy literature, with original observations done 25 years ago. Thunderstorm asthma has occurred four times in Melbourne, Australia, over the last 30 years, and is also reported in Wagga Wagga, NSW, Australia (once) with isolated reports in the UK, USA and Europe.
Here is an ABC Catalyst report on this condition from 2011.
Why does it happen?
It occurs when three factors combine:
- A susceptible population
- A high pollen source
- The correct weather conditions
What is the susceptible population?
South-East Australia is prone to some of the worst spring hay fever in the world. It occurs generally from September to the end of December, with documented highest pollen levels from mid-October to mid-December. Its two major cities, Melbourne and Canberra, have the most severe seasons on the continent. Melbourne has 4.5 million people and 20% (900,000) have hay fever. Another 900,000 would have positive allergy tests for pollens but do not show symptoms. About 50% of those with spring hay fever have either chronic persistent asthma through the year, or seasonal asthma only in spring, or mild chest symptoms during spring that is only apparent by direct questioning (personal observations and published studies). The other 50% have no clinical asthma symptoms at all.
We also know that up to 65% of people with hay fever, but without asthma, have irritable airways due to the pollen allergy. This is only detectable by special tests in respiratory clinics (nonspecific bronchial hyper responsiveness).
Pollen appears to be the important allergen, but there are a few studies implicating mould spores or pollutants.
Where is the high pollen source?
The southern temperate pasture grasses are planted throughout most of Victoria and in southern NSW. There are about 12 grasses that have similar allergens (so-called cross-reacting). The leading candidate is perennial ryegrass (Lolium perenne), and others include canary grass, cocksfoot, timothy, meadow fescue, and redtop among others. These grasses are planted mainly for feed (as hay). There is a contribution from weeds and the main weed is Bermuda grass (called couch grass [pronounced ‘kooch’] in Australia). Bermuda grass also has some cross-reacting allergens, but also at least one unique allergen. The pollen released is massive, amounting to tonnes per hectare.
What are the correct weather conditions?
The pollen-containing winds in Melbourne are Northerly and North-Westerly. The winds pick up the pollen and carry them south to 4.5 million people. As locals know (especially sailors!) these winds are gusty and can be very strong. They can swing about, and the winds are often called flukey by sailors.
To worsen the problem, there is a gap in the relatively low mountain range north of Melbourne. This gap is around the town of Kilmore. It is called the Kilmore gap by pilots and anecdotally acts as a Venturi suction funnel, concentrating the pollens as they head south.
So far you have described conditions that occur every spring in Melbourne. What sets the thunderstorm asthma conditions apart?
The studies by the late Prof Bruce Knox at the Botany School, University of Melbourne, in the 1980s, and by others, established that moisture may cause the pollen granules to explode, releasing hundreds of subpollen particles (sometimes called submicronic particles). Pollen itself can cause allergies in the eyes and nose, and secondary asthma in susceptible people, but generally is too large to enter the tiniest lung airways. The subpollen particles swiftly enter the lung and are breathed into the smallest tubes. Therefore a further trigger is moisture and this occurs with rain.
These subpollen particles are also highly allergenic.
But during spring in Melbourne, it’s often not just rain. It is a storm, usually (but not always) from the South-West. And the storms can be violent, with high winds. Furthermore, there is evidence that these cold gusts force a downdraft (I speak as a layman here, not a meteorologist) and the downdraft catches debris, insects and pollen, extending these in a concentrated line in front of the storm front.
How does this allow us to predict likely risk situations?
In the southern hemisphere the high weather system travels anti-clockwise. As the centre of the high crosses Melbourne, the winds turn northerly. That is how the weather bureau predicts the next day’s pollen count.
If there is a long tail to this northerly airstream, sometimes from central Australia, loosely called a tropical airflow, then more pollen will be picked up, and there may be some moisture in the air already. If there is an approaching ‘cool change’ (often welcomed by Melburnians at that time of the year), then this is shown by a cold front, usually with fairly close isobars, approaching from the south-west.
These are the conditions that are high risk.
There are many variables and future research should analyse all these variables and create a suitable model.
Can high risk days be predicted?
Yes. AllergyNet Australia has tweeted about high risk days for the last 5 years. Thankfully, all days passed without incident except the most recent.
Can an Thunderstorm Asthma event be absolutely predicted?
No. We need research to develop a prediction model.
Is there an example that you can show?
On the evening of Monday 21st November 2016 Melbourne experienced a severe and lethal Thunderstorm Asthma event. Media reports are here and here.
These are the weather maps for that day and the days before and after:
How many high risk days are there?
There are usually 4-6 high risk days between mid-October to mid-December.
What are the main research aims?
- Develop an effective prediction model
- Identify risk factors in those with spring hay fever but without clinical asthma
What is your advice with the knowledge we have now?
- If you get spring hay fever, and have persistent asthma through the year, or if you have asthma in spring, or if you have cough or chest tightness or feel ‘chesty’ in some springs but not all, review an asthma action plan with your doctor. Asthma, even occasional, during spring suggests that a daily preventer spray from September to the end of the year is warranted. Make sure you have antihistamines and Ventolin* (both available over the counter) with you. *or equivalent rescue/reliever puffer
- Activate your asthma action plan early.
- Remember: For sudden severe asthma in spring, take 4 puffs of Ventolin, wait 4 minutes, if no better take another 4 puffs, wait 4 minutes, if no better call an ambulance (call 000). Take two antihistamines even if you have had one that day (this is quite safe). Suitable antihistamines are fexofenadine (Telfast or generics), loratadine (Claratyne or generics), or cetirizine (Zyrtec or generics).
- Remember: Control your hay fever through spring with daily antihistamines if needed. Think about a referral to an allergist in autumn. Allergen immunotherapy is effective for hay fever, and reduces the risk of asthma. For immunotherapy, there must be no contraindications, supportive positive tests, and suitable supervision.