The soupy blanket of volcanic haze that rolls in and settles over Honolulu's skyline when trade winds slow or stop can be just miserable.

But, experts say, it's not dangerous.

Even the thicker vog haze O'ahu has seen since March 2008, when emissions from Kīlauea doubled, don't have concerning levels of sulfur dioxide or fine particulates (as they do on the Big Island).

And there is no evidence to suggest the vog seen on O'ahu or the other small islands in the chain can trigger long-term health problems.

That doesn't mean the vog doesn't affect otherwise healthy people.

On voggy days, many on O'ahu complain of watery eyes, scratchy throats and headaches. Asthmatics and others with respiratory diseases report feeling short of breath, and are sometimes forced to stay inside.

More than 175,000 people in the Islands have some type of lung disease. "For many of them, a vog day is very, very difficult," said Jean Evans, executive director of the American Lung Association of Hawai'i.

She added the association is increasingly getting phone calls from people without lung diseases complaining about the effects of the vog. "Their eyes get red and itchy. Their throat gets scratchy," Evans said.

Part of the problem is that though the vog O'ahu gets is not considered dangerous, it is sometimes thicker — with more fine particulates — since Kīlauea emissions increased two years ago.

On a very voggy day in 2008, Honolulu air monitors recorded a 24-hour average of 34 micrograms of fine particulate per cubic meter of air, which is just shy of federal air quality warning levels. The annual average for 2008, though, was just 4.7 micrograms per cubic meter.

By comparison, Honolulu failed federal air quality standards for particulates on Jan. 1, 2010, because of fireworks smoke, with monitors measuring a 24-hour average of 62 micrograms per cubic meter.

Meanwhile, the Big Island community of Pāhala, about 18 miles downwind of Kīlauea, saw a 24-hour high of 76 micrograms of fine particulate per cubic meter in 2008 because of volcanic emissions.

CONDITIONS WORSEN

At very high levels, fine particulates can aggravate heart and lung diseases, triggering heart attacks, asthma attacks, respiratory symptoms and bronchitis, according to the Environmental Protection Agency.

The particulates in vog are tiny, just 2.5 micrometers and smaller.

That's about 1/30th the diameter of a human hair.

The emissions that Kīlauea spews have significant concentrations of sulfur dioxide, but not necessarily high levels of particulate. Most fine particle pollution is formed as volcanic emissions react with oxygen, dust particles, water and other chemicals in the air.

The vog that makes its way to O'ahu has very little sulfur dioxide, said Donald Thomas, the director of the Center for the Study of Active Volcanoes at the University of Hawai'i-Hilo. It's mostly fine particulates.

Tamar Elias, a chemist at the U.S. Geological Survey's Hawaiian Volcano Observatory on the Big Island, said the chemical changes that vog undergoes highlight the importance of having air monitoring stations around the Islands, not just at the source of emissions.

"The vog is a brew," she said. "It's one thing to measure the emissions coming out of the volcano. But it's something else to measure the components that are affecting people" in different communities.

And exactly what that brew is made up of changes constantly.

MAJOR IRRITATION

Still, many say the vog O'ahu gets — even with low levels of particulates — can be a mighty irritation. And some point out that O'ahu is not only seeing thicker vog, it's seeing more voggy days.

The state Health Department said there were at least 43 days on O'ahu last year when volcanic haze was so thick that officials ordered no agricultural burning, so smoke wouldn't further worsen air quality.

In 2008, there were 61 such days on O'ahu.

There were 24 "no burn" days on O'ahu in 2007, the year before Kīlauea emissions increased, and 32 "no burn" days on O'ahu in 2006.

Beth-Ann Kozlovich, director of development at the Lung Association of Hawai'i and an asthmatic, said the vog may not pose an immediate health threat to people on O'ahu, but it does make for uncomfortable days. "It's just unpleasant for anybody," she said.

Even so, many Big Island residents, especially those downwind from Kīlauea, cringe when people from Honolulu complain about the vog.

Jessie Marques, volunteer executive director of Ka'ū Rural Health Association, said the only time Big Island residents get a respite from volcanic haze is when the trade winds stop or slow, and the vog drifts to Honolulu. "When Honolulu talks about vog," she said, "we're living it."