Health advice has been issued as ash from the volcanic eruption in Iceland fell towards the ground in some parts of the UK.
Particles were expected to reach ground level in Scotland first before moving south.
The Health Protection Agency (HPA) said the ash should not cause serious harm but could cause irritation, particularly to those with respiratory conditions.
It advised people with conditions including asthma and chronic bronchitis to carry inhalers and medication with them and those who noticed symptoms, including itchy or irritated eyes, runny nose, sore throat or dry cough, should either return to their homes or "limit their activities outdoors".
But the Met Office said any ash which did reach ground level would be barely visible and the public should not be concerned.
The HPA said weather patterns indicated a "small fraction" of the volcanic plume in the UK's airspace, which could include low levels of sulphur dioxide, was likely to reach the ground.
In a statement, the HPA said: "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."