Distinctive dust on the bonnets of cars across the city has been confirmed as volcanic ash from Iceland after going under the microscope at Sheffield Hallam University.

Scientists from the University's Materials and Engineering Research Institute (MERI) collected samples of the ash, which has grounded flights to and from the UK, on Friday morning and examined them in their laboratory.

Dr Hywel Jones, consultancy manager for MERI, first spotted the dust, which the Department of Health says is not dangerous, on his cycle ride into work.

He then asked a research student to collect small samples from cars parked in Sheffield Hallam's Science Park.

Hywel said: 'It was a curiosity factor for me to see if this dust was volcanic ash. We analysed the samples and found they contained silicon and oxygen, calcium, aluminium and sodium, which make up volcanic matter.

'It is essentially volcanic rock that has been melted and frozen in the atmosphere.'

Hywel said it was incredibly rare for a sample like the volcanic dust to be collected in Sheffield.

He said: 'It is certainly unusual. We have examples of Sahara sand sometimes covering cars but this is clearly a different sample.'

The World Health Organization said it was unclear what exact health risks there would be from the ash cloud but Europeans should try to stay indoors if ash from Iceland's volcano starts raining down from the sky. The updated UK recommendations come after reports of a small concentration of particles reaching the ground in Scotland.

People may notice a dusty haze in the air, the HPA said, but any health effects are likely to be short-term. They also said that low levels of sulphur dioxide are likely to be found in the plume but this is also not expected to be a threat to human health.

If people smell sulphur, rotten eggs, or a strong acidic smell, when outside they may wish to limit their activities outdoors or return indoors. Anyone with respiratory conditions such as chronic bronchitis, emphysema and asthma should ensure they have any inhalers or other medications with them, the recommendations said.