One of them was Eritrean-born Mohamed Basher, 33, who went into cardiac arrest after enduring a severe asthma attack that left him unable to breathe. Mr Basher woke up seven days later in the intensive care unit and spent five weeks in St Vincent's Hospital after developing a lung infection. He said he didn't have allergies when he moved to Australia in 2001 but started getting itchy eyes and hayfever a few years later. "I try to stay indoors as much as I can during October and November," he said. Many people were outside on that day in November when a cool change swept through the city, bringing relief to residents after a run of hot weather but also saturating the air with invisible and unprecedented danger.

High winds caused fields of ryegrass in Melbourne’s north and west to release their pollen into the warm spring air. The sudden gust of colder, moister air then took that pollen and shattered it into millions of tiny fragments which entered the lungs of asthmatics around the city. The disaster that followed is currently the subject of a coronial inquiry, which heard this week that among those who suffered the most were Asian migrants — many of whom developed their allergies only after moving to Australia. A study by a team of Melbourne scientists presented to the Coroner’s Court found that 39 per cent of the patients who flooded emergency rooms were of Asian or Indian background and 54 per cent of those admitted to intensive care were born overseas. Ten people died during the disaster, six of them born in India or other parts of Asia. "People of Asian background coming to Australia, a very typical pattern is the first five years they are OK, between five and 10 years they start getting hayfever, and then they develop wheezing and asthma," says one of the study’s authors, Professor Francis Thien, who works with migrants every day as an asthma specialist at Eastern Health.

He points to a piece of research published in 1996, which found that Asian migrants were almost twice as likely to have hayfever as people born in Australia; the longer they lived in Australia, the more likely to develop the condition they became. As many as six in 10 would develop spring hayfever within three years. That paper was written by Dr Roland Leung, who first noticed that his fellow international students in Melbourne were developing hayfever soon after arriving in Australia. While the findings broke new ground, Dr Leung is disappointed a follow-up investigation hasn't uncovered more. "Should it have been done, it might have prevented some of the thunderstorm asthma attacks in subsequent years," he says. One theory is that migrants don’t encounter the same allergens, particularly ryegrass pollen, in their home countries. When they come in contact with such pollen in Australia, their allergies are activated.

Dr Naghmeh Radhakrishna, based at the respiratory department at St Vincent's Hospital, says

there are migrants who are developing symptoms of hayfever or asthma but aren't picking up on it. "A lot of the time they're not on appropriate medication or they aren't using their treatment properly, and their asthma is not particularly well controlled," she says. "They're at a much higher risk." While the thunderstorm asthma epidemic most affected Asian migrants, there are signs the issue could be spread much wider. A 2003 paper looked at a broader range of migrants attending high school in Western Sydney, finding that those who moved from a country with low asthma rates (which includes Asian countries, parts of Africa and the Middle East) developed an increase in symptoms the longer they were in Australia. Perhaps most startling was that the symptoms flared within two years.

One of the study's authors, Professor Peter Gibson, was also involved in separate research which found that Greenland Inuits who moved to nearby Denmark had higher rates of asthma than those who didn't. He is unable to explain why such an effect takes place but suggests that the current best guess by researchers is changes in lifestyle, particularly diet. "It could be adopting a western diet, which is high in saturated fat, low in soluble fibre and low in anti-oxidants," he says. The last thing Mr Basher remembers on that spring day in 2016 before he slipped into unconsciousness is telling his wife to call an ambulance. Paramedics had to put a tube into his lung to help him breathe. Despite the sudden onset of the attack, at least he knew he had asthma — many of those affected had no idea.