Much of south-east England was braced last night to experience black toxic rain as oily soot from the burning of almost 16m litres of oil, aircraft fuel and petrol started to fall over the region and the plume of gases reached ground level in some areas.

According to the Meteorological Office, a cold front was expected to stretch across the region, bringing rain and forcing the pollutants from the fires down to ground level. The intensity of the fires throughout most of yesterday, combined with very light winds and a high pressure system, carried much of the pollution 2,750 metres (9,000ft) above an area stretching at one point from East Anglia to Salisbury Plain, in Wiltshire.

Health experts warned that the elderly, asthmatics and anyone with respiratory problems should take great care not to inhale the fumes, which they expected to worsen as the fires weakened and were unable to force the smoke so high.

Professor Warren Lenney, of the British Lung Foundation, said the consequences of inhaling soot particles from the acidic thick black smoke could be "unpleasant".

"The trouble is there are all sorts of different chemicals in the smoke. Petroleum products are known to produce a whole series of nasty acidic chemicals, as well as carbon monoxide." Prof Lenney, consultant paediatrician at the University Hospital of North Staffordshire, said side-effects could include coughing, difficulty breathing and a lack of concentration as oxygen levels in the brain dropped.

A police spokesman said: "We are advising people to close their windows and doors and to go immediately to a doctor if they breathe in the fumes."

The potential effect of the explosion was being assessed by the Environment Agency last night. The agency said the main areas of concern were the potential for water pollution and damage to land quality. A spokesman said that would only occur if any of the substances, including kerosene, diesel, gas oil and gasoline, were to escape from the site.

Colin Chiverton, of the agency, said: "At the moment we are closely monitoring the situation for any potential environmental impact and will continue to do so."

A spokeswoman for Total UK, the British arm of the US oil company which half owns 60% of the site, said petrol and kerosene, as well as some diesel and crude oil were held there, but she did not know exactly what had caught fire. "We understand air samples have been taken and they show the toxicity to be low," she said.

Staff at Hemel Hempstead general hospital said extra beds and intensive care facilities had been set aside for anyone suffering smoke-related complications.

Dr Ivan Vince, a health, safety and environmental specialist for ASK Consultants, said he believed the smoke plume was not so dangerous at present because it was being lifted high above the ground and would be greatly diluted before it returned to ground level. But he warned: "When the fire is dying, the smoke is less buoyant and so can cause a local hazard."