The extent of wildfire damage in Northern California is revealed in this 21 October image from NASA’s Terra satellite. Vegetation appears red; burned areas are dark gray. The body of water at top right is Lake Berryessa. Credit: NASA

Over the weekend of 8 October, wildfires broke out across the wine country north of San Francisco. Thousands evacuated and dozens died. Carried by the wind, an 800-kilometer trail of smoke snaked across the landscape, poisoning the air for millions of people living in the Bay Area.

For hours, even as the air quality in San Francisco began to visibly deteriorate, the Environmental Protection Agency’s air quality index (AQI) for the city was green, indicating normal, healthy conditions. Warnings eventually went out, but they were frustratingly vague. Were the particulates in the smoke-filled air the only danger, or were there toxic gases as well? The AQI offers no way to tell.

Introduced in 1999, the AQI standardizes public health information about the air we breathe. But between delays in reporting and excessive simplicity, the AQI doesn’t adequately inform people of how to respond to changing conditions. The experiences of Bay Area residents during the recent wildfires illustrate the need for more detailed and transparent public health messaging.

Quantifying bad air

The AQI encompasses ground-level ozone, fine and coarse particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Using thousands of monitoring stations across the country, the EPA measures individual pollutant concentrations at 1-, 8-, or 24-hour intervals. A calculation converts each measurement into an AQI value. The final AQI that is shared with the public, along with its corresponding color (see the table below), is the highest of the six calculated values. The AQI is “the highest level of the pollutant that is of most health concern that day,” says Aaron Richardson, a public information officer with the Bay Area Air Quality Management District.

Credit: EPA

The result of that methodology is an index that’s easily digestible but dangerously unspecific. An AQI warning looks the same no matter which pollutant triggers it. For example, air quality deemed orange (AQI 101–150) is considered unhealthy for sensitive groups. Yet who is part of a sensitive group changes depending on the pollutant (see table 3 in this EPA document). If the trigger is carbon monoxide, people with heart disease are most at risk. If the trigger is ozone, the list of sensitive groups includes children and the elderly. In addition, the duration of exposure necessary to induce symptoms and the symptoms themselves change for each pollutant. Overall, the simplicity of the AQI makes it difficult for people to understand their personal risk and make informed decisions about how to mitigate it.

Fire factor

With fires, the most important pollutant tracked by the AQI is particles. Fires produce particulate matter in the form of organic aerosols made of carbon, hydrogen, and oxygen, says Shawn Urbanski, a researcher at the US Forest Service’s Missoula Fire Sciences Laboratory who investigates the chemistry of smoke. Fine particles create a major public health concern because they can be inhaled deep into the lungs.

Particulate matter is classified into two categories: fine particles, just a fraction of the width of a human hair at less than 2.5 μm in diameter (PM2.5); and coarse, larger particles, up to 10 μm in diameter (PM10). Fine particles are produced by vehicle traffic, fires (tamed and untamed), and industrial processes. High levels of fine particles are usually more of a concern during the winter, when people are burning wood for cozy fires at home. Cold, stagnant air traps the pollution in the lower atmosphere, smothering valley towns.

California’s unusual fires Fire season is a normal part of life in the western US and western Canada. But this year’s fires have an added fuel source: grasses and thick shrubs that surged in growth during the long, wet winter and then dried out over the summer. Once a fire gets started, shrubs enable “high-intensity spreading over a large area quickly,” says Allie Weill, a fire ecology researcher at the University of California, Davis. Topography is also enhancing the fires. Fire climbs even the steepest of hills quickly through a combination of convective and radiative heating along the slopes, Weill says. Strong Diablo winds—offshore katabatic winds that heat up as they sink— fan the flames, spreading the fires farther. That recipe for fire may become more common due to climate change. The western US and western Canada experience an annual summer drought that shortens in length as you move up the coast: Southern California’s lasts up to 10 months, while temperate rain forests in British Columbia experience just a few weeks. “With climate change, it appears that this drought is going to expand,” says Eric Taylor, an air quality meteorologist with the government of British Columbia. As the period of drought grows, so will the fire season.

But all bets are off once a wildfire starts. “If a fire burns a thousand acres in a single day, it’s producing more PM2.5 emissions than all the automobiles, fuel combustion, and industrial processes that day,” says Urbanski. Despite strong winds blowing the pollution plumes out over the Pacific Ocean, the deterioration of air quality during the California fires shattered records. “We have seen lots of colors in the bay,” says Richardson, referring to the AQI’s rainbow of warnings. “This was a completely anomalous situation.” On 10 October the AQI for Napa hit 472, which corresponds to more than 18 times the daily fine particle safe exposure limit established by the World Health Organization.

Those AQI warnings didn’t always reflect what was happening with the air outside, however. Due to the measuring of pollutants on different time scales, it took 24 hours for fine particle counts to transition from healthy green to unhealthy red, even when people were choking on the visibly miserable air. The lag time would have been even worse in a less populated area, where air-monitoring stations are sparsely positioned or absent altogether.

Further, the AQI didn’t adequately warn Bay Area residents of lesser but still significant pollutants in the air. Throughout October, nitrogen dioxide and ozone levels have intermittently reached the yellow range—signifying a moderate health concern—even as the AQI reflected solely the PM2.5 count. Yellow levels of ozone can be enough to trigger respiratory problems for the extensive population that is particularly sensitive.

What next?

Local governments may be leading the way to something better than the AQI. The Bay Area Air Quality Management District, for example, reports the AQI values for individual pollutants along with detailed public health messaging, rather than cramming everything into a single composite warning. Agencies in the region have also begun building temporary monitoring stations to better track fluctuations in fine particle counts due to fires in rural areas. On a national level, the EPA is acknowledging the stresses posed by wildfires by running a separate notification system for impacted communities with high fine particle counts.

The lesson in those efforts is that timely, specific messaging is the way to go when it comes to air pollution. Let’s abandon opaque indexes and report pollution concentrations directly.