London, England (CNN) -- The ash from the volcanic eruption in Iceland this week is paralyzing air travel because it is too dangerous for aircraft to fly through it. Those on the ground may be concerned about health effects when the ash falls to Earth, but experts say there is little to worry about.
Britain's Health Protection Agency said the concentration of ash particles that may reach the ground "is likely to be low and should not cause serious harm."
Ash is made of fine particles of fragmented volcanic rock. It often has small pieces of lava or cinders. It can have acid coatings that can cause lung, eye and minor skin irritation.
According to the International Volcanic Health Hazard Network, in most eruptions, volcanic ash causes very few health problems. In fact, officials there say, there is "almost no risk to people" from this particular ash eruption.
Any effects people do feel are likely to be minor. People may experience itchy or irritated eyes, a runny nose, sore throat or dry cough, or they may notice the smell of sulphur or see a dusty haze, the Health Protection Agency said. Eyes can become painful, itchy, or bloodshot or produce a sticky discharge, and gritty pieces can scratch the cornea, causing abrasions or a non-contagious form of pinkeye. Skin irritation is less common, but if the ash is acidic, skin could redden and become irritated.
Any such health effects are likely to be short-term, the British health agency said. And any concentration of volcanic dust on the ground is likely to be low and not pose a significant threat to public health, the agency said.
But those with pre-existing respiratory problems could experience bronchitis or asthma-type symptoms like wheezing and shortness of breath because fine ash particles can irritate airways, causing them to compress, or they can cause the lining to make more secretions inducing coughing and heavy breathing.
People with chronic illness such as asthma, bronchitis, emphysema or heart disease are at greatest risk. The International Volcanic Health Hazard Network says long-term exposure to fine volcanic ash can lead to even more serious lung disease.
Ash on the ground is a problem in the immediate vicinity of the volcano in Iceland, where authorities are warning people to use masks and protective goggles when outside. Those with respiratory problems should remain inside, Icelandic civil emergency authorities said Friday.
But Iceland is receiving a considerable amount of ash on the ground, those authorities said, and that's likely to dissipate after the ash crosses the ocean into other countries.
"There's no chance at all that'll affect human health," said Iarla Kilbane-Dawe, innovations director at AEA, an environmental consultancy in Britain.
"Even if it did, by the time it reached us, it would become very, very diluted. In the 2,000 kilometers or so it's traveled coming from Iceland to the UK, it will have spread over hundreds of thousands of cubic kilometers of air and become extremely diluted indeed."
Kilbane-Dawe said the AEA monitored air pollution overnight Thursday and hasn't seen any "major spikes" of pollution from the volcano, even in places in the United Kingdom -- one of the closest European countries to Iceland -- where people are already smelling the ash.
Falling volcanic ash is "more bothersome than hazardous to your health," according to the World Health Organization. It advises people to stay calm when ash is falling and to stay indoors. The agency recommends closing the doors and windows, and sealing off chimneys.
"If people are outside and notice irritation in their throat and lungs, a runny nose or itchy eyes, they should return indoors and limit their outdoor activities," says Dr. Maria Neira, director of public health and environment department at the WHO.
Because ash is pulverized rock, a one-inch layer of dry ash weighs 10 pounds per square foot. As long as the ash stays in the upper atmosphere, there will probably be no escalation in health risks, according to the WHO.
"Particulate matter is identified according to its diameter. The small particulates less than 10 microns in size are more dangerous because they can penetrate deeper into the lungs," Neira said. "Since the ash concentration may vary from country to country depending on the wind and air temperatures, our advice is to listen to local public health officials for the best guidance for individual situations."
If there are no effects on human health, could the ash pose any problems -- or benefits -- for the land? Kilbane-Dawe said gardeners should not hold out any hope of a boost to their plants.
"It's much too diluted by the time it gets to your garden to really help," he said.
The ash is likely to increase the acidity of the soil, but whether that helps plants is determined by whether those plants prefer lime-filled or acidic soil.
CNN's Saundra Young contributed to this report.