Thunderstorm‐related respiratory symptoms: An old story with a new ending?
Authors: Francis Thien, Isabella Annesi‐Maesano, Gennaro D'Amato
Journal: Allergy (2024)

On November 21, 2016, the world was shaken by a catastrophic epidemic thunderstorm asthma event which overtook the Melbourne, a city of over 5 million people in south-eastern Australia. Following heavy early seasonal spring rains which allowed a lush growth of vegetation, a 10 day long hot dry spell in late spring was broken by a cool change and thunderstorms crossing over the city at 6 pm. A large proportion of the unsuspecting population was mainly outdoors, enjoying the beaches to cool off, or commuting on the way home from work. Within hours, hospital emergency departments were inundated as thousands of people presented with symptoms of respiratory distress and ambulances were overwhelmed with distress calls for help. The official figures of 3365 excess public hospital emergency department presentations with respiratory-related cases and 476 excess asthma-related hospital admissions (a 7-fold and 10-fold increase above average respectively) underestimate the thousands of others who presented to general practitioners and community pharmacies seeking relief.1 This dramatic event brought into sharp relief the story of thunderstorm-related respiratory symptoms which have been described globally now for over 40 years. Initially reported in the United Kingdom in 1983, dozens of events have been reported in diverse geographical regions including Canada, USA, Italy, Germany, Saudi Arabia, Iran, Kuwait, Northern China, Australia and New Zealand.2 The Melbourne 2016 event was the deadliest by far with 10 fatalities, with 5 fatalities reported subsequently in Kuwait in 2016, and previously reported events (in UK and Australia) with single fatalities (Table 1). What have we learnt and how can we be better prepared to prevent or mitigate the risk of future events? These thunderstorm respiratory events arise from a common complex interaction of environmental and individual susceptibility factors. Main environmental factors include high concentrations of an aeroallergen and the presence of thunderstorms. Allergens which have been described include various mould species (Didymella exitalis, Myxomycetes and Alternaria sp.), weed (Parietaria/Urticaceae sp. and Amaranthaceae sp.), mugwort (Artemisia sp.), olive tree (Olea europaea) and ryegrass (Lolium perenne) pollens.3 Thunderstorm characteristics include outflows with colder air associated with the down draft from the thunderstorm, sweeping up pollen grains and particles and then concentrating them in a shallow band of air bringing atmospheric aeroallergens to ground level. For ryegrass pollen triggered events, evidence suggests thunderstorm moisture and rainwater ruptures ryegrass pollen grains by osmotic shock, releasing fine (<2.5 μm) allergen-bearing starch-granules respirable to the lower airways. This allows bypass of the usual upper airway filter and trigger acute bronchospasm via a putative direct lower airway allergen challenge, even in those not previously known to be asthmatic. Individual risk factors include prior allergen sensitization and exposure, with the presence of rhinitis being both a marker of allergic sensitization as well as a risk factor for latent and undiagnosed asthma. In those with diagnosed current asthma, low adherence to preventive inhaled steroids increases thunderstorm asthma risk. Other factors more recently identified include Asthma Control Questionnaire (ACQ) score >1.5, ryegrass pollen specific IgE > 10.1 kU/L, raised eosinophil counts, and raised fractional exhaled nitric oxide (FeNO) level.4 The effect of climate change on allergenic pollen and weather patterns and their impact on health and healthcare services is a burgeoning area of concern. Increasing atmospheric carbon dioxide (CO2) concentrations and temperatures have important effects on plants, their pollen production, and the consequent burden of atmospheric allergens.5 Seasonal variations and increases in temperature affect the timing of the pollen season for some plants, including starting pollen release earlier and lasting longer. In addition, significant increases in pollen allergenicity have been reported as associated with increasing CO2 and temperature. Climate change can also increase the likelihood of extreme weather events, such as thunderstorms and unseasonal rainfall promoting luxuriant plant growth, alternating with hot dry spells promoting pollen release.5 Air pollution that is also influenced by climate change significantly exacerbates the risk and severity of thunderstorm asthma by interacting with pollen, enhancing respiratory sensitivity, and contributing to a pro-inflammatory state in the lungs.2 Understanding this relationship is crucial for preventing and managing thunderstorm asthma, particularly in urban areas where pollution levels are high. Improved understanding of these risk factors for thunderstorm asthma leads to consideration of the following preventive strategies at the individual, public health and environmental level. For individuals with asthma, access and adherence to an inhaled steroid/fast onset long-acting bronchodilator (ICS/LABA) medication (such as a budesonide and formoterol combination inhaler) during the pollen season should be promoted. Those with allergic rhinitis need to be treated with preventive therapy such as intranasal steroids, with consideration of, and access to ICS/LABA inhalers especially if there are any respiratory symptoms suggestive of latent or undiagnosed asthma. Allergen specific immunotherapy (if available) can also be considered as primary or secondary preventive risk reduction strategies.6 At the public health level, pollen monitoring can facilitate pollen forecasting and warning systems, to allow pollen sensitive individuals to stay indoors and avoid exposure to harmful pollen levels. In the Australian state of Victoria, following the Melbourne 2016 event, a thunderstorm asthma forecasting system during the pollen season has been implemented which combines meteorological and grass pollen forecasts to produce daily forecasts of low, moderate, or high thunderstorm asthma risk. During high-risk days with high airborne pollen concentrations (>50 pollen grains/m3) and severe thunderstorms with strong winds, public health messages are promoted through public and social media to stay indoors, and for those at risk with asthma and allergic rhinitis to have available and take their inhaled/intranasal medication. Health services are also put on alert for thunderstorm asthma outbreak response preparedness. These strategies need to be tailored for particular geographical regions' health system resources and thunderstorm asthma risk profile (Figure 1). Lastly, climate change is a global health concern at multiple levels. There is an urgent need to adapt to and mitigate the effects of climate change on human health. Emissions reduction and air quality improvement (through reducing fossil fuel use and increasing environmental biodiversity and green spaces) and promoting sustainable agricultural practices will help to reduce the impact of climate change.7 The paradigm and catastrophic potential of epidemic thunderstorm asthma emphasises the need to take urgent action in order to write a new ending to an old story. None. The authors have no conflicts of interest to declare related to this manuscript. Data sharing is not applicable to this article as no new data were created or analyzed in this study.