The microscopic particles which make up volcanic ash pose a threat to aircraft because they can affect visibility and get sucked into aircraft engines, causing them to shut down.

Yesterday, airports across the country, which had been able to handle some early-morning arrivals and departures before the flight ban, effectively shut down from noon - and passengers were warned to expect more disruption today.

The ash, from the Icelandic mountain Eyjaffjalljokull, also caused airport and aircraft movement shutdowns in other parts of Europe, including France, Sweden, Finland, Denmark and Holland.

Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office, said: "It is likely that the production of ash will continue at a comparable level for some days or weeks.

"But where it disrupts travel, that depends on the weather. It depends how the wind carries the ash."

It was not the first time air traffic has been halted by a volcano, but such widespread disruption has not been seen the September 11 terror attacks.

The Health Protection Scotland and NHS National Services Scotland warned last night that some of the ash would reach ground level.

A statement read: "It is important to stress that the concentration of particles which does 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. Health Protection Scotland, the Health Protection Agency and the Met Office will continue to monitor the situation and issue any further advice or updates as necessary as the weather changes."

But the Met Office said any ash that does reach ground level will be barely visible and the public should not be concerned.

Met Office forecaster John Hammond said there had always been a small chance of ash 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."

And he said that, although visibility "dropped away for a short time" in the Shetland Islands, this was "unlikely" to happen further south.

The dust is "unlikely to cause any long-term health effects", he added.

"The amount of dust in the air is not exactly known but we do know which regions have been affected and the effects on health will be relatively small," he said.