Fire crews at the Buncefield oil depot inferno are nearing the final phase of a massive operation as they fight to put out fires in three blazing tanks.
They are tackling fires in the last two smaller tanks still ablaze at the Hertfordshire terminal, before moving on to the last and largest tank.
The fire - which has been blazing since Sunday morning - should be out by the end of the day, fire chiefs have said.
Police say evacuated families may be able to start returning to their homes.
Meanwhile Deputy Prime Minister John Prescott has been holding a meeting of the Cobra civil contingencies committee to discuss the events at Buncefield.
Smoke and soot
People living near the Hemel Hempstead depot are still being urged to stay inside with their doors and windows closed, to minimise any chances of suffering ill effects from the huge smoke billowing from the site.
But a police spokesman said earlier he expected the "main risk" to be over by the end of the day.
A Met Office spokesman said the wind had switched from south-westerly to northerly on Tuesday, pushing the smoke plume from the fire towards the south.
There are fears soot could cause problems at ground level as the plume falls to earth once the blaze has been put out.
At a press conference on Tuesday morning, Assistant Chief Constable Simon Parr of Hertfordshire Police said: "As the blaze is tackled, the smoke plume will start to fall to the ground in slightly greater density."
Assistant Chief Constable Parr said he hoped residents of dozens of still empty houses near the site could start returning on Tuesday afternoon.
All schools closed because of the oil blast will reopen on Wednesday, apart from a very few which were structurally damaged, Hertfordshire County Council said.
Dr Jane Halpin, from the local health authority, said there appeared to be no significant increase in people reporting breathing difficulties.
FIGHTING THE BLAZE
Six high-pressure pumps
250,000 litres of foam concentrate
15m litres of water
30km of hose
26 fire engines
20 support vehicles
75% of Hertfordshire fire personnel on site at least once
Support from 16 other brigades
But NHS Direct said its Hemel Hempstead call centre had received about 50% more calls than usual on Sunday and Monday because of the fire.
The centre normally gets about 100 calls a day, but on the day of the explosion around 150 people rang in, and the next day the figure was 160.
The extra calls were all from people regarding smoke inhalation and difficulty breathing, with 10 pregnant women ringing in concerned about their unborn babies.
The blaze began just after 0600 GMT on Sunday, when the first of a series of explosions hit the site.
Forty-three people have been injured.
One person admitted to Watford General Hospital intensive care with respiratory problems remains in a stable condition.
A public meeting for residents has been taking place at Hemel Hempstead sports centre.
It follows a similar meeting held on Tuesday morning for businesses inside the police cordon which have been directly affected.
At a meeting on Monday evening, some of those evacuated were warned it could be a week after the fire was put out before they were able to return home.
This was because their homes would have to be assessed for potential contamination and structural problems to make sure they were safe.
A police advice line for affected residents and businesses has been set up on 0800 0960095. Oil firm Total, which operates the depot, has also set up a hotline - on 0870 400 0499 - to enable those whose properties have been damaged to log their details.
Police say there is "nothing to suggest" the fire was anything other than an accident.
And a spokesman for Total denied there had been leaks in the run-up to the explosions.
The Buncefield depot is a major distribution terminal operated by Total and part-owned by Texaco, storing oil and petrol as well as kerosene which supplies airports across the region, including Heathrow and Luton.