Last week was notable for the heat index getting close to or above 100 degrees Fahrenheit in some parts of the state. Yet, another unpleasant condition and human health irritant was noticeably absent: vog.

With a general lack of tradewinds Thursday, vog could have made an appearance. But Kilauea volcano on Hawaii island is no longer emitting high levels of the gas that produces vog.

“We’re barely able to measure any vog right now,” said Lacey Holland, a postdoctoral fellow who runs a vog measurement and prediction project at the University of Hawaii’s Department of Atmospheric Sciences. “Everybody should get out and enjoy the good air quality.”

For about a month now, emission rates of sulfur dioxide (SO2) that mixes with moisture and other particles in the atmosphere to produce volcanic smog otherwise known as vog have been at lows not recorded since late 2007, according to the U.S. Geological Survey’s Hawaiian Volcano Observatory.

HVO reports that SO2 production from Kilauea has been under 1,000 metric tons per day since late August. On Thursday, Holland reported SO2 output at just 200 metric tons.

Those figures compare with about 5,000 metric tons a day on average during the last decade up until May 3 when lava erupted from Kilauea’s lower East Rift Zone near Pahoa and caused SO2 levels to soar, at times reaching more than 50,000 metric tons a day in July, according to HVO.

Lava flows ceased late last month, reducing output of the poisonous SO2 gas that had driven many residents away from Pahoa and produced vog that drifted to other parts of Hawaii County and neighboring islands.

‘Absolutely clear’

Greg Ogin said clear skies in West Hawaii, where vog often traveled on prevailing tradewinds, have been refreshing.

“It’s kind of like a breath of fresh air,” he said. “It’s just absolutely clear.”

Ogin, who lives above Keauhou south of Kailua-Kona, said the recent air quality reminds him of the time before Kilauea’s Puu Oo vent opened in the East Rift Zone in 1983 and began emitting lava and SO2. Before that eruption, he recalled, distant parts of the island such as the slopes of Mauna Loa to the south were clearly visible and sunsets were more beautiful. “I had almost forgotten,” he mused.

Tim Lucas, a retired educator in Honolulu who for years took school groups on visits of Kilauea, said a generation has grown up with vog in Hawaii. “That was just something we lived with,” he said, adding that its absence is one good thing that followed the destructive eruption.

Coltin Kendig of Kealakekua on Hawaii island said he would have expected to see vog Friday with returning tradewinds if the eruption hadn’t petered out. During times of heavy vog, he felt like he had a “muddy lung” that made it a little harder to breathe. Now, he admires distant views and clean air.

“We’ve been able to see all the way down the coastline to South Point,” Kendig said.

Unpredictable condition

How long vog-free conditions last can’t be predicted because SO2 levels can fluctuate even without an active eruption and scientists said they can’t determine how long Kilauea may go without erupting again.

SO2 exists in magma deep underground where pressure is high, but gets released into the air when lava reaches the surface where low pressure allows the gas to escape. It has a sharp pungent odor that HVO geologist Janet Babb said can smell like the ignition of a match or firecracker.

The gas can be an irritant to the skin, eyes, nose and throat but also can cause more serious problems including respiratory distress. Additionally, some people report headaches, energy loss and flu-like symptoms due to SO2. During the recent eruption, many people sought medical treatment for reactions to SO2.

Plumes of the gas in the air mix with moisture and other particles to create vog that regularly travels to West Hawaii with prevailing tradewinds. In the absence of tradewinds, vog can waft over East Hawaii and other islands.

The hazy volcanic air pollution may have negative health impacts on people far from an eruption, and some people stay inside to avoid breathing the dirty air. Sulfuric acid in vog also can damage plants and corrode metal when brought down to the ground as acid rain.

Vog became a frequent problem in 1986 when lava from Puu Oo changed from intermittent to near constant production, according to HVO. For more than a decade since that time, SO2 emissions averaged around 2,000 metric tons a day during sustained eruption periods.

Babb at HVO said the main “degassing” vent for SO2 changed from Puu Oo in 2008 when a lava lake formed at Kilauea’s summit. When the lake formed, SO2 from Puu Oo dropped to a few hundred metric tons a day and the summit began emitting 5,000 metric tons a day on average.

Then in May right before the latest eruption, the summit lake drained down and lava broke out of numerous ground fissures near Pahoa and covered more than 6,000 acres of land while also destroying more than 700 homes. For the last several weeks, hardly any SO2 has been detected from mobile sampling of areas around the recent eruption zone, while a few hundred metric tons has been measured at Kilauea’s summit and at Puu Oo.