Juneau is no longer meeting federal air quality standards. Not because we're polluting more, but because the standards have tightened.
Those standards say when air is safe to breathe, and they changed recently after scientists learned how harmful fine particles can be.
"Over the last decade there's been a growing body of evidence that the old PM2.5 standard is not protective of public health," said Clint Farr, a state air quality manager at the Department of Environmental Conservation.
PM2.5 refers to fine particles 2.5 micrometers or smaller across - about ⅓0 the width of a human hair - suspended in the air. The EPA issued tighter standards in 2006, and states and municipalities are still learning what that means for them.
The U.S. Environmental Protection Agency last week proposed listing Juneau and Fairbanks as in "nonattainment" of its fine particulate standard, which is based on a three-year average of daily readings.
Juneau areas haven't violated EPA standards since the 1980s, when the Mendenhall Valley still had a lot of dusty unpaved roads. Paving them took care of that problem, Farr said. Since then, Juneau's had officially healthy air. But a few too many bad-air days in the valley last winter pushed it into noncompliance with the EPA.
It's not just bureaucratic regulation. These particles cause heart disease, lung cancer, asthma and death, according to scientists. They're just the right size to lodge in human lungs. Their composition varies; they may be relatively harmless dust, but they may also be chemicals that interact with your respiratory tract in a nasty way. One study estimated that between 22,000 and 52,000 people died nationwide in 2000 from breathing fine particulate.
Juneau's wet weather has its benefits: It scrubs out air pollution.
"We have some of the cleanest air on the planet - for the most part," Farr said.
Except in the Mendenhall Valley, in the winter, on those days when the air is stagnant.
The valley is, as far as local air quality trackers know, its own airshed. It has a lot of people, a lot of whom have wood stoves that emit those fine particles. And in the winter, valley air occasionally is subject to inversions. When warm air sticks around near the ground instead of rising as usual, wood smoke doesn't rise either, and people end up breathing what they burn.
They're relatively short and rare for Juneau, but inversions can set up for "weeks on end" in Fairbanks, Farr said.
"We have problems all over the (Pacific) Northwest in the winter," said the EPA's Krishna Viswanathan.
A subtlety of the new designation is that EPA is considering expanding the "nonattainment" designation to all of Juneau.
That's because valley wood stoves may not be the only source of PM 2.5, according to Viswanathan. It also can be formed in the atmosphere from other kinds of pollution, such as car exhaust. And the fine particles can travel much farther than the larger 10-micrometer ones that EPA used in the old standard.
EPA's waiting for more data from local monitors to decide whether just the valley or all of Juneau is to be listed as having unhealthy air. A final decision is expected for December.
Planning for clean air is the city's responsibility. The Assembly this week updated its code to manage for the EPA's new standards.
When the valley forecast looks like an inversion, the city may issue an "air alert" or "air emergency" for the area. An air alert advises people to avoid using their wood stoves. An air emergency bans them, except for pellet stoves, and it's a misdemeanor to violate it.
Violators, beware your neighbors.
"People see the smoke," said city staffer Michelle Brown. "It's voluntary compliance, but their neighbors will report them. They'll call the police, just like anything else."