LONDON had been hot and dry for days, and a thick layer of smog mingled with pollen hung over the city. As the thunder clouds rolled in on 24 June 2005, the UK's Health Forecasting Unit put hospitals on standby and issued a health warning: all hay-fever sufferers, even those with no previous history of asthma should stay indoors or risk a severe asthma attack.

The authorities had good reason to be concerned. On the same day 11 years earlier, identical weather conditions had conspired to make pollen and spores more allergenic than usual, and "thunderstorm asthma" struck thousands of people, overwhelming the city's emergency rooms. This time the hospitals were more prepared. Despite having up to eight times as many patients admitted with asthma attacks as usual, they had had some warning of the influx, and the public had clear advice on what preventive measures they should take.

The ...