People with lung conditions like emphysema and asthma should stay indoors if volcanic ash starts to settle, a spokesman for the World Health Organisation said today.
The ash from Iceland's volcano has potential health consequences because some particles may reach deep into the lungs, he said.
"From what we know at the moment, the majority of cloud ash is seven to 10km high up in the atmosphere," said spokesman Dan Epstein.
"Air quality monitoring networks have not yet reported particulate on the ground but, if it does reach the ground, it may have health effects.
"People with existing respiratory conditions such as asthma, emphysema and bronchitis should avoid breathing in these particles.
"They should have their inhalers and other medication with them."
Mr Epstein said particles measuring less than 10 microns have the potential to reach lower airways and cause problems in some people.
Therefore those with lung conditions should stay indoors if the ash starts to settle, he said.
However, this did not mean everyone should go indoors and disrupt their routines, he said.
A spokesman for the Health Protection Agency (HPA) in the UK said Mr Epstein's advice was in line with its own.
He said the situation was being monitored closely and could change but that regular updates were being sent out.
The current advice is for people with respiratory conditions to ensure they had their medications, and for all people to consider going inside if they begin to notice effects that could be related to the ash.
A statement said: "It is important to stress that the concentration of particles which may reach ground level is likely to be low and should not cause serious harm.
"If people are outside this evening and notice symptoms such as itchy or irritated eyes, runny nose, sore throat or dry cough, or if they notice a dusty haze in the air or can smell sulphur, rotten eggs, or a strong acidic smell, they may wish to limit their activities outdoors or return indoors.
"Those with existing respiratory conditions such as chronic bronchitis, emphysema and asthma may notice these effects more than others and should ensure they have any inhalers or other medications with them. Any such health effects are likely to be short-term.
"The Health Protection Agency, Health Protection Scotland and the Met Office will continue to monitor the situation and issue any further advice or updates as necessary as the weather changes."
Met Office forecaster John Hammond said: "There's always been a small chance of it reaching the ground.
"Over the next few days or so, with winds as they are, there is a chance we will see some small deposits but these will be quite difficult to see.
"It might be easiest to see anything that comes out of the sky on cars because the amounts will be very small."