A cloud of black smoke is spreading over a large area of southern England after a series of explosions at an oil depot in Hemel Hempstead.
Although winds are light, smoke is drifting towards Reading and Swindon, the Met Office has said.
The smoke is likely to affect only people who already have respiratory illnesses, health officials say.
Jane Halpin, Hertfordshire's director of public health, said some people could experience coughing or wheezing.
She told a news conference that the smoke was likely to affect only people with illnesses such as asthma and chronic obstructive pulmonary disease. Those most at risk were people who had inhaled it.
Dr Halpin said work was under way to analyse the content of the smoke.
"We have got work in hand to sample the smoke to be clear about exactly what's in it and what it's made up of.
"And really we need to know more about this before we can understand what long-term implications there may be, if any, from this exposure," she said.
Dr Pat Troop, Chief Executive of the Health Protection Agency (HPA), said the smoke contained hydrocarbons, which is what is found in soot.
She said it was the small particles in the smoke that cause the irritation.
"If it's only a short amount [the person is inhaling] then it won't do any long-term harm," she told the BBC News website.
She added the HPA would be monitoring the situation and will be looking at any ongoing health implications.
"Dealing with fires is not unusual, it's just the sheer size of it that is different," she said.
The Met Office said north-westerly winds on the surface were extremely light, but higher up north-easterly winds were stronger. The smoke could reach Southampton by Sunday night, it said.
Smoke has risen very high with aircraft reporting it at an altitude of 9,000 feet.
Peter Kidds, a weather forecaster for the Met Office, said sooty particles could fall to the earth as the smoke cools.
They would contain pollutants, not in high concentrations, but enough to "be of concern".
He added that for at least 24 hours there was not expected to be any change in the weather, including wind directions, that would affect the smoke cloud.
The Environment Agency said it was assessing the environmental impact of the blasts.
It said the main areas of concern were potential pollution to groundwater, rivers and streams and damage to land quality.
It said this would only occur if substances such as kerosene, diesel, gas oil and gasoline were to escape from the site.
There was no evidence to suggest this had happened so far but the situation was being closely monitored, it said in a statement.
Howard Borkett-Jones, medical director at Hemel Hempstead General Hospital advised those who felt the effects of smoke to "reduce their exposure to it by keeping all their doors and windows closed".
He said although the smoke was of "low toxicity", hospital staff were prepared for dealing with some respiratory problems.
"We understand that in general terms it is of low toxicity but people might experience some irritation to their respiratory system, some slight tightness in the chest because of irritation and coughing as a result of exposure to noxious chemicals in the atmosphere," he said.
He added if people continue to feel unwell they should contact NHS Direct or their GP's out of hours service.
A police spokesman said people should keep out of the smoke and to keep doors and windows shut.
"This cloud, not least because it contains heavy smoke, is an irritant, and will certainly make people who inhale it potentially cough, potentially irritate the eyes and potentially feel nauseous.
"Our very strong advice is that people must keep out of the plume of smoke."
He added people whose windows have been blown out should evacuate to "somewhere safer".
People experiencing symptoms of smoke inhalation can contact NHS Direct on 0845 4647.