The volcano fallout is unlikely to pose a major health risk, but people should watch out for symptoms including itchy eyes or a sore throat, said Health Protection Scotland.
"Updated information on weather patterns in the UK now indicates that volcanic ash associated with the current eruption in Iceland will reach ground level over the UK, starting in Scotland this evening before moving south over the course of the night," said a Health Protection Scotland statement.
"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."
But it said: "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 ... they may wish to limit their activities outdoors or return indoors."
The British Met Office however downplayed the risk, saying any ash that did fall to the ground would be barely visible.
"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," said Met Office forecaster John Hammond.
"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."
Fallout from the Eyjafjallajokull volcano in southeast Iceland threw up a huge cloud of ash across northern Europe, prompting authorities to close the airspace in Britain and at least seven other countries.
He added WHO were still trying to gather more information on the exact health risks associated with the ash cloud.
WHO said there was no set concentration threshold that the ash cloud must have to make it a health risk.
"Any particular matter that is deposited (and) breathed into the lungs is dangerous to people," said the spokesman. "It depends on how the particles are suspended in the cloud and their density. We don't know that yet."
But health officials in the UK appeared to play down the concerns, saying a 'small fraction of Icelandic volcanic plume' would fall to ground level in Britain tonight and tomorrow morning.
European weather experts said the ash cloud was spreading southwards over Europe and eastwards into Russia at the speed of a slow moving car.
