What is the smoke made of?
Mostly it comprises soot and some toxic elements such as carbon monoxide, hydrochloric acid, oxidised nitrogen and acrolein.
Large particles of soot are not usually regarded as a health risk because they get trapped in hairs in the nose and airways. Smaller particles get deep into the respiratory system, where they cause health problems.
Doctors are most concerned about PM10s - particulates smaller than 10 microns, or a hundredth of a millimetre.
But the high temperatures have burnt much of the potentially dangerous elements.
How dangerous is the smoke?
Anyone exposed to high concentrations for an hour or more could have problems. However, there is no evidence that people were caught in dense, choking fumes at the time of the explosion.
Prof Warren Lenny is the spokesman for the British Lung Foundation and an expert in child health at University Hospital of North Staffordshire NHS Trust. He said: "Potentially even small amounts of these compounds can cause irritation in people who are sensitive. The nose, the mouth and the airways can become itchy and irritated.
"If the irritation is severe it can cause swelling of the tubes and lead to breathing difficulties."
Those most at risk include the elderly, asthmatics, people with chronic lung disease and the infirm.
"The police and the health service gave good advice, which was to stay indoors," said Prof Lenny. "Although it must still be unpleasant in the vicinity, it is dissipating, rather than building up. It should be normal in a number of days."
Any serious health problems would have shown up in accident and emergency departments on Sunday and Monday. But health officials in the Hemel Hempstead area report "very low levels" of people with breathing problems or who are seeking advice.
If people exposed to the smoke did not experience any problems at the time, they were unlikely to do so now.
Prof Stephen Spiro, of the British Lung Foundation, said: "We anticipate that there will be little lasting damage to the lungs of those exposed to the recent smoke pollution.
"Most of the oil was kerosene and the particles are generally small, light and should disperse high into the atmosphere and therefore will not prove a health hazard."
What if you are affected?
"The treatment for smoke inhalation is basically to give people oxygen," said Prof Lenny. "If people experience irritation then they should take a soothing cough medicine or suck a sweet. It would not be my advice to go to the doctor.
"If you are normally well this should not be a problem. If you have asthma then you should take your preventive medication, which you should be taking anyway, and then the medication to relieve symptoms as you would in any circumstance where you feel something will trigger an attack.
"If you have a severe chest problem there is even more reason to stay indoors."
Will conditions get worse?
The fixed air pollution monitors around London and the South East are recording no noxious chemicals more than 10 miles from the fire.
Within 10 miles people are advised to stay indoors if they are at risk. Schools have been advised to close. However, as the fire comes under control and starts to smoulder, the temperature at the depot will drop - generating low-level smoke that could pose more of a risk to locals.
The larger particulates dropping on cars, washing lines and the ground pose a "minimal risk", said the Helath Protection Agency. It is unlikely that they could be inhaled if disturbed, but they may contain some irritant chemicals. They can be safely washed off. If it rains, more soot could be brought to the ground.
Again, it is likely to be mostly carbon, although it could contain some toxins.
Could food be contaminated by particulates?
Most dairy cattle are indoors at this time of year, eating hay cut in the summer. The risk to milk supplies is low, said the Food Standards Agency. Locals are advised to wash and peel fruit and vegetables exposed to soot.
Could the water supply be contaminated by the fire fighting?
The Environment Agency says it is happy that foam and water sprayed on to the fire will not contaminate the water supply. The run-off is being contained by specially constructed dykes and pumped to storage facilities at the site.
Firefighters are drawing water from a lake.
As a precaution, drinking water bore holes have been closed in an operation involving Three Valleys Water.