The Met Office said ash that reaches ground level will be barely visible
Ash from the volcanic eruption in Iceland that has started falling on Scotland's Northern Isles "should not cause serious harm", the NHS has said.
The eruption under a glacier in Eyjafjallajoekull area is the second in Iceland in less than a month.
The ash has already reached ground level in Shetland and is expected to move south overnight.
A statement from the NHS said people with respiratory conditions should keep medication with them.
Health Protection Scotland and National Services Scotland said: "If people are outside 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."
It might be easiest to see anything that comes out of the sky on cars because the amounts will be very small
The statement added: "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."
Residents in Shetland and Aberdeen have already reported smelling the effects of the ash, describing it as "sulphurous".
Dust collected in Lerwick, Shetland, is being analysed by the Scottish Environment Protection Agency.
The Met Office said ash that does reach ground level will be barely visible and the public should not be concerned.
Met Office forecaster John Hammond: "There's always been a small chance of it reaching the ground. It happened over the Shetland Islands for a time this afternoon when we had some very small deposits of dust.
"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 was "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.
Finance Secretary John Swinney updated the Scottish Parliament on the situation in an emergency statement in the afternoon.
He said: "The analysis that we have of emissions from volcanic eruptions is that the ash is not poisonous and has the potential only to irritate those who may suffer from skin conditions or asthmatic conditions."
There had been no indication that ash should cause alarm to farmers, he added.
From Democracy Live: Emergency ministerial statement on the volcanic ash cloud.
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