Smoke billows from the Eyjafjallajokull volcano in Iceland on Wednesday. European health officials are warning people to stay indoors when the ash starts to settle. (Ingolfur Juliusson/Reuters)

People with breathing problems should take precautions if air quality worsens across northern Europe from ash particles following the eruption of a volcano in Iceland, the World Health Organization said Friday.

The health risks as a result of Wednesday's eruption, and the resulting cloud of microscopic particles, are still being determined based on when the cloud settles, said WHO spokesman Daniel Epstein.

"Any particulate matter that is deposited, breathed into the lungs is dangerous to people, so we are concerned about that, but we don't have details yet," Epstein told a briefing in Geneva.

The Icelandic volcano grounded aircraft across the continent. The cloud of particles is drifting above 6,000 metres and is invisible from the ground.

"We do have a lot of experience with levels of particulate matter and only if [levels] were very high they would be of concern," Dr. Carlos Dora, co-ordinator of public health with the WHO in Geneva, said in an interview with CBC News.

European health authorities are looking for high levels of particles on the ground before they issue any health warnings, Dora said.

Monitor air quality

European cities have very good air quality monitoring, so are equipped to monitor conditions and advise people, he noted.

Health Canada's website also noted that microscopic volcanic ash particles can be inhaled deeply into the lungs. Respiratory problems depend on several factors, the department said:

• How many and how big the particles were in the ash that was inhaled.
• How often and how long the ash was breathed in.
• If volcanic gases were breathed in.
• The weather conditions.
• If a person already has heart and lung problems.

British health officials expect a "small fraction of the Icelandic volcanic plume" to reach ground level in Britain on Friday night and Saturday morning. Low levels of sulphur dioxide are expected in the cloud.

If air quality conditions deteriorate, the advice for people in northern Europe with respiratory problems, asthma, or cardiovascular problems are the same as those as when smog alerts are issued:

• Avoid intense exercise.
• Keep inhalers and medication with you.
• Avoid exhaust from heavily trafficked roads.

"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," Britain's Health Protection Agency said, adding that any health effects from the ash cloud are likely to be short term.

"Not all particles are created equal," said Ken Donaldson, a professor of respiratory toxicology at the University of Edinburgh who has studied the impact of volcanic ash in people. "In the great scheme of things, volcanic ash is not all that harmful."

Mask advice 'bit hysterical'

Aside from those directly near the Icelandic volcano, most Europeans would have neglible exposure to the volcanic ash since the particles get diluted in the stratosphere, Donaldson said.

As particles travel, they get more diluted and less dangerous, agreed Dr. Stephen Spiro, a professor of respiratory medicine and deputy chair of the British Lung Foundation.

The cloud has already passed over northern Scotland with no reports of ill effects, Spiro said.

Advice to wear masks or stay indoors to avoid volcanic ash was "over the top" and "a bit hysterical," Spiro said.

"If this was really coming down, you'd see a yellow [tinge] in the air from the sulphur," he said. "But we've seen no sign of that."

If people experience symptoms such as itchy or irritated eyes, a runny nose, sore throat or dry cough, or notice a dusty haze in the air or smell the rotten egg odour of sulphur outside, then "they may wish to limit their activities outdoors or return indoors," Dora said.

The circumstances are different for people in Iceland who are close to the volcano, where larger, course particles may reach the ground.

In that specific case, people would be advised to stay indoors, not wear contact lenses, and wear goggles and masks, Dora said.

With files from The Associated Press