'Suddenly my eyes and throat started burning': what caused Birling Gap's toxic cloud?

Last August, holidaymakers in East Sussex fell ill after a poisonous yellow cloud spread across the sky. What was it, and where did it come from?

Mark Sawyer has worked for the Royal National Lifeboat Institution for nearly 30 years, and since 2001 he's been the full-time coxswain at the Eastbourne lifeboat station. Shortly after 5pm on the Sunday of a bank holiday weekend last August, he received a report from the coastguard in Southampton about an incident at a beach seven miles west of his station. "The call we got was that there had either been a fire or an explosion at Birling Gap, and they'd got 50-plus casualties suffering from smoke inhalation or burns." There was what looked like a layer of thick smoke hanging just above the sea.

Birling Gap is a popular National Trust spot between Beachy Head and Seaford, a dip in the chalk cliffs of the Seven Sisters, with a steel staircase leading down to a pebble beach. At low tide there is sand and rockpools; on the cliffs above there is a visitor centre, cafe, car park and coastguard station.