Icelandic Met Office's Matthew Roberts: 'The ash cloud reached 8km high'
Experts are reassuring people that the ash cloud over Europe poses no health risk beyond areas close to the volcano.
Air quality monitoring shows ash pollution isn't arriving at ground level in most of Europe.
The World Health Organization advice ties with what UK experts have said - that small amounts of ash are unlikely to cause serious harm.
As a precaution, the Health Protection Agency advises people to stay indoors if they get respiratory symptoms.
If people notice symptoms such as a runny nose, itchy eyes or cough they may want to go inside, says the HPA and Scottish advisors.
Those with conditions such as asthma may notice the effects more, they said.
WHO spokesman Carlos Dora said on Tuesday that the ash plume may thin out high up in the air and no longer be a problem.
We would advise people living with a lung condition in affected areas to carry their medication as a precaution as they may experience a short-term worsening of symptoms
Professor Malcolm Green, British Lung Foundation
Dora says the most dangerous ash particles are the smallest ones which can be breathed deep into the lungs, and which have moved further from the volcano site in the ash plume billowing over Europe.
"The deeper they penetrate into the lungs, the bigger their potential concern for public health," he said.
The UK recommendations came after reports of a small concentration of particles reaching the ground in Scotland.
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.
The British Lung Foundation backed the updated advice.
Spokesman Professor Malcolm Green said: "We would advise people living with a lung condition in affected areas to carry their medication as a precaution as they may experience a short-term worsening of symptoms."
The first eruption occurred near the Eyjafjallajokull glacier, in Iceland. The glacier water melted by the ongoing volcanic activity in the region generated large volumes of steam, which made this latest eruption explosive.
The Met Office is monitoring the plume of volcanic ash which has grounded flights across the UK.
Dust and odours were detected in the Northern Isles and the dust is being analysed by the Scottish Environment Protection Agency.
Scientists have pointed out that, at ground level, volcanic ash can cause serious health problems.
Dr Dougal Jerram, a volcanologist from the Department of Earth Sciences at Durham University explained: "One of the most influential ever eruptions was the
1783-1784 event at Laki in Iceland
when an estimated 120 million tonnes of sulphur dioxide were emitted, approximately equivalent to three times the total annual European industrial output in 2006.
"This outpouring of sulphur dioxide during unusual weather conditions caused a thick haze to spread across Western Europe, resulting in many thousands of deaths throughout 1783 and the winter of 1784."
Following that event, many people reported seeing a "volcanic haze" near the ground. The current ash plume is not visible from ground level.
This was also a much smaller eruption and scientists have said that this is a relatively diffuse ash cloud that will blow away within one or two days.
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