29/10/2025
The Lancet: Epidemic Thunderstorm Asthma on Nov 21, 2016, Melbourne (VIC, Australia) was
experiencing the peak of an unprecedented spring heatwave.
Temperatures that day climbed to 35°C, the hottest recording
since March that year, and the pollen count was extremely
high, with airborne ryegrass pollen concentrations of more
than 100 grains per m3 of air. Around 1400 h, the Australian
Bureau of Meteorology issued warnings across most of
the state for severe thunderstorms with damaging winds.
Between 1700 h and 1830 h, the temperature suddenly
dropped from 35°C to the low 20s and thunderstorms started
to erupt, sending severe gust front winds over Melbourne’s
metropolitan area. Within an hour, in a nightmarish scenario,
the emergency medical services started to receive hundreds
of calls for acute respiratory distress and breathing difficulties
throughout the state. By midnight they had received calls
for 1326 cases, a caseload so extreme that they ran out
of ambulances after attending to 500. The call volume
remained above normal levels until 0700 h the next day.
Within 30 h of the storm breaking, there were 3365 excess
respiratory-related presentations to emergency departments
(ie, 672% above the average), and 476 excess asthmarelated admissions to hospital (992% above the average).
Additionally, there was a substantially increased number of
out-of-hospital cardiac arrests and prehospital deaths. In
total, around 10000 people needed treatment in hospital
emergency departments for asthma attacks within a short
time of the thunderstorm and ten people died, six within a
week of the storm. A representative of Ambulance Employees
Australia commented to a newspaper that Nov 21 “would
have been a traumatising night…an event equivalent to a
terrorist attack where people are critically injured”.
The 2016 Melbourne event was the largest and most
catastrophic occurrence of epidemic thunderstorm asthma,
defined as multiple presentations of acute asthma attacks
or bronchospasms immediately after a thunderstorm.
The event was not unique; although rare, there have
been 26 epidemic thunderstorm asthma events reported
globally since 1983. 11 of these events have happened in
Australia, mostly in the Melbourne area, but thunderstorm
asthma events have also been documented in Saudi
Arabia, North America, Iran, China, UK, and Italy, with
epidemic events noted in 1994 in London (UK), in 2013
in Ahvaz (Iran), and in 2016 in Kuwait. The most recent
thunderstorm asthma event was recorded in Yulin (China)
in 2018.
Because thunderstorm asthma events have occurred
most frequently in the state of Victoria (Australia),
policymakers there (and in other regions with a history of
thunderstorm asthma) have taken proactive measures to
manage the harmful outcomes of epidemic thunderstorm
asthma, including creating a website that forecasts the risk
of an event occurring within the next 3 days and issuing
news alerts warning of the imminent possibility of epidemic
thunderstorm asthma. The forecasts are based on a risk
matrix identifying the meteorological and environmental
conditions involved in thunderstorm asthma (such as
high concentrations of an aeroallergen), combined with
identification of biological risk factors and susceptible
individuals, according to those most affected in previous
events. The Victoria risk forecast website deems the highest
risk of an epidemic thunderstorm asthma event occurring
between Oct 1 and Dec 31 (the ryegrass pollen season)
on days when there is a high pollen forecast and severe
thunderstorms with strong winds are likely to be present, and
that people with current, past, or undiagnosed asthma or hay
fever are most at risk, especially those with both conditions
or poorly controlled asthma. This risk prediction is generally
accurate regarding the basic data from the 2016 Melbourne
event—a high pollen count coincided with a thunderstorm
that day, and the 2018 inquest heard that all ten people
who died had pre-existing asthma. However, the causes and
risk factors underlying epidemic thunderstorm asthma are
more complex than initially thought. High pollen counts
and thunderstorms often occur together, but don’t always
trigger asthma events. Ryegrass pollen grains are normally
too large to reach the lungs and usually get no further than
the upper airways after inhalation. The role of thunderstorms
was originally thought to centre around high humidity
conditions causing pollen rupture and release of sub-pollen
particles that are small enough to be easily inhaled deep into
the lungs. However, the relative humidity during events such
as the 2016 Melbourne event was very low with little rainfall,
thus humidity-induced rupturing cannot explain the 2016
Melbourne event. A model hypothesised that alternative
pollen rupture mechanisms, especially those caused by
lightning strikes, might have generated the pattern of a large
release of sub-pollen particles in the 2016 Melbourne event
following the storm’s path; however, the correlation between
the lightning strikes and asthma attacks is not perfect and
many other triggers of thunderstorm asthma will need to
be uncovered in future studies. Ryegrass pollen might be the
main allergen involved in thunderstorm asthma in Australia,
but other allergens play a role in thunderstorm asthma
events in other countries, such as Parietaria pollen in an event
in Naples (Italy) in 2004 and mugwort pollen in the 2018
Yulin event. Concentrations of other allergens such as fungal
spores are also hugely increased during thunderstorms.
Identifying individuals who might be at risk of epidemic
thunderstorm asthma is just as complicated. Although
all ten individuals who died in the 2016 Melbourne event
had a previous diagnosis of asthma, some reports suggest
that current asthma was not a sufficient predictor of
risk of a thunderstorm asthma attack during the 2016
Melbourne event, with only 28% of 2242 cases presenting....... Gennaro D'Amato commented "Considering climate change with increasing prevalence of thunderstorms and increasing atmospheric concentrations of pollen grains due to global warning, it is possible that in future there will be an increasing frequency of thunderstorm asthma. It is important to focus the attention of clinicians on these events, which are frequently still underestimated."
