Helicopters could not be used to investigate a chemical haze on the Sussex coast last summer as it was not known if the gas was explosive.
Birling Gap was evacuated and more than 150 people needed hospital treatment.
The National Police Air Service (NPAS) and the Ministry of Defence (MoD) were asked for help by Sussex emergency teams, but they said their aircraft could not fly, documents show.
East Sussex fire service used gas detection units, but found nothing.
People reported burning eyes, sore throats, skin irritation and vomiting after the gas engulfed the beach on the August bank holiday weekend.
Source: BBC News