San Francisco is used to conditions that are dusky, dense and altogether soupy.

These are the kind of hazy gray skies — cruelly prevalent in the summertime, when "clear" and "blue" are the most yearned for words in Bay Area meteorological forecasts — that causes many newcomers to squint, sigh and shake their fists at the heavens. Seasoned San Franciscans, however, are accustomed to and even relish the murky atmospherics. After all, in how many other cities can the vision-obscuring phenomenon that occurs when cool marine moisture mingles with hot inland temps be described as world-famous? How many other cities have fog that actively tweets?

What San Franciscans aren't accustomed to are similarly opaque conditions in which they're urged to stay indoors or don a respirator mask if they do venture out. And this is what the city has been grappling with since late last week: a layer of toxic smog — not the usual gloomy but benign fog — that's blanketed the Bay Area as the Camp Fire, a ruinous blaze of historic proportions, continues to rage over 150 miles away in Butte County. (As of the this writing, the wildfire is now 60 percent contained having already scorched nearly 150,000 acres per Cal Fire.)

In fact, San Francisco's decimated air quality has garnered a number of its own headlines.

This isn't to detract from the unprecedented tragedy still unfolding in the fire zone, but the smoke that has drifted from the northeast and settled over the Bay Area has resulted in air quality so compromised — officially categorized as "unhealthy" or "very unhealthy" by the U.S. Environmental Protection Agency — that it poses its own distinct dangers. It's so bad that San Francisco's iconic cable cars have been temporarily taken out of service, schools and universities have been shuttered and Uber drivers are distributing filter masks to passengers.

Even the fog is sending out tweets urging people to stay indoors.

Air pollution in California cities tops India, China

As reported by Quartz, San Francisco had the worst air pollution of any major city in the world on Nov. 15 according to air quality monitoring firm AirVisual, beating out the perilous-to-breath-in Asian metropolises — particularly those in India and China — that typically top global air quality indexes in a not-so-good way.

The following day, San Francisco dropped to number two on AirVisual's rankings with an air quality index (AQI) value of 259, second to only Dhaka, Bangladesh with its certifiably hazardous rating of 449. Index values over 151 are considered "unhealthy" by the EPA while anything over 201 is considered "very unhealthy." Other major cities with exceptionally bad air quality on that date include Lahore, Pakistan; Ulaanbaatar, Mongolia; New Delhi, India and the Nepali capital of Kathmandu. Needless to say, San Francisco's presence on this list is unprecedented and alarming.

"It appears to be the worst air quality ever experienced in San Francisco," Dan Jaffe, a professor of environmental chemistry at the University of Washington, says of the ongoing "air quality emergency" to CNN.

Stanford University obscured by wildfire smoke Wildfire smoke obscures the sun over Stanford University in Palo Alto, California. (Photo: Jason O. Watson/Getty Images)

It's worth pointing out that there's been some confusion about whether San Francisco truly reached "worst in the world" AQI status at any point since the Camp Fire first began — potentially sparked by a faulty power line operated by investor-owned utility PG&E — on Nov. 8. (This is the same day the smaller Woosley and Hill brush fires, the former receiving a significant amount of media attention, both broke out in Southern California.)

As Curbed explains, it's likely that San Francisco's air quality, while indeed record-breaking-ly awful as Jaffe notes it was, wasn't technically the worst in the world when considering AQI data from other sources including the Bay Area Air Quality Management District (BAAQMD), which notes that nearby cities including Oakland and San Pablo had higher levels of polluted air than San Francisco proper on Nov. 15.

What's more, Robert Rohde, lead scientist at Berkeley Earth, notes that typically smog-enveloped New Delhi and other Indian cities with off-the-charts AQI data just happened to be having an unusually good day at the same time things were deteriorating in Northern California.

"The winds are light and offshore, so there's nothing to move the smoke," National Weather Service meteorologist Suzanne Sims told the San Francisco Chronicle. "It's there, and it's not going anywhere."

Hazardous air that shifts with the wind

Nearer to the Camp Fire and its difficult-to-fathom path of devastation, the AQI value in Sacramento — as measured by the EPA's AirNow air quality map and forecast tool — topped out at around 316 late last week while Chico, to the north, reached hazardous heights of 437. As of this writing, Sacramento's AQI value has been downgraded to an "unhealthy" 179. In Chico, where hundreds off evacuees from the fire are living in makeshift tent cities, the current value hovers around 230.

The air quality in Stockton, a mid-sized inland port city situated along the San Joaquin River roughly 50 miles south of Sacramento, has also been deemed "very unhealthy" over the past week with no signs of letting up.

San Francisco shrouded in wildfire smoke San Francisco is often cloaked in fog, but 'very unhealthy' air quality is a new problem. (Photo: Justin Sullivan/Getty Images)

Away from major cities in small towns located close to the fire, index values are reported to have reached 470, which almost goes beyond the AQI limits established by the EPA. (Anything over 500 is treated in the same way as the public health crisis-level "hazardous" category, which is 301 and above.) Per AirNow forecasting data, Stockton along with Modesto, Yuba City/Marysville, Chico and Gridley are expected to have the highest AQI values in the nation in the coming days.

Given the unpredictable way in which smoke from large wildfires can disperse across a region, cities in the Bay Area have had higher AQI values than communities located closer to the fire during certain periods. This has obviously since changed although San Francisco (current AQI value: 135) isn't out of the woods quite yet.

According to the American Lung Association, California is already home — catastrophic wildfires aside — to some of the most polluted metro areas in the country when ranked by short-term particle pollution, which is what the state is currently dealing with in a most extreme manner. Bakersfield tops the list followed by Visalia-Porterville-Hanford, Fresno-Madera and Modesto-Merced. All are located in the heavily agricultural San Joaquin Valley.

Paradise, California, during Camp Fire Plumes of hazardous smoke fill the skies above Paradise, California, on Nov. 8. (Photo: Justin Sullivan/Getty Images)

Famously smoggy Los Angeles — currently forecasted to enjoy a "moderate" air quality index value of 58 — ranks at number seven. It is, however, number one in the country when it comes to ozone-based air pollution. Fairbanks, Alaska, suffers from the worst year-round particle pollution due to burning of wood and other fuels during its epically long winter. Driving home the fact of how grim things are in California at the moment, Fairbanks' current AQI value is a "good" 28.

"We have made tremendous efforts and investment to clean up our air with considerable benefits for public health," Dr. Daniel Jacob, a professor of atmospheric chemistry and environmental engineering at Harvard University, tells CNN. "But now it's like we're getting stabbed in the back with those wildfires."

Small particles pose huge public health threat

While most forms of air pollution carry health risks, air quality compromised by wildfire smoke is particularly dangerous due to the presence of tiny and insidious particulate matter (PM) formed by the combustion of wood and other organic compounds. Along with carbon monoxide, nitrogen dioxide and ozone, air quality indexes are largely based on the presence of particles measuring less than 2.5 micrometers in diameter or PM 2.5. Larger particulate matter — or PM 10 — refers to airborne irritants such as pollen and dust.

According to the AirNow website: "These microscopic particles can penetrate deep into your lungs. They can cause a range of health problems, from burning eyes and a runny nose to aggravated chronic heart and lung diseases. Exposure to particle pollution is even linked to premature death."

Butte County, California, power lines and wildfire The area of Butte County where the worst wildfire in California history was sparked. (Photo: Josh Edelson/AFP/Getty Images)

Although healthy adults have less to worry about when it comes to short-term exposure to air with elevated PM 2.5 levels, the risks are higher for children and teens, the elderly, pregnant women and those suffering from existing respiratory ailments as well as those with heart disease and diabetes.

As Vox notes, breathing in heavily polluted air like the kind currently found in Sacramento or Chico without any kind of respiratory protection throughout a full day is roughly equivalent to smoking an entire pack of cigarettes or more.

In areas with elevated air pollution levels, authorities are urging residents to stay indoors, which, in reality, doesn't completely eliminate exposure to PM, particularly in mild San Francisco where much of the building stock, particularly residential, is older and not equipped with air conditioning systems that circulate and filter air. (Quartz points out that this is the main reason schools and universities in San Francisco and neighboring cities cancelled classes late last week.) Jaffe of the University of Washington tells CNN that those living and working in buildings with functioning air filtration systems can "reduce their PM exposure by about 90 percent."

Respirator mask at wildfire victim vigil A respirator mask hangs at a vigil held for wildfire victims in Chico, California. (Photo: Noah Berger-Pool/Getty Images)

For Californians venturing outdoors, experts recommend the use of N-95 or 9-100 respirators, both of which are special masks found at most home improvement stores. (While they may block larger irritants from entering airways, snuggly wrapped and/or damp bandanas and scarves along with run-of-the-mill paper surgical or dust masks won't cut it with smaller particulate matter.) Jaffe and others stress that the masks must achieve a perfect fit (translation: be tightly sealed) to be effective. People with facial hair who attempt to wear the masks are doing so in vain.

Not surprisingly, shortages of the masks, have been reported across wildfire-impacted areas of California. So-called "Air mask selfies," which, per the Washington Post "capture the gravity of the hazardous conditions" in the Bay Area, are also apparently a thing now.

Breathing easier with rain in the forecast

At a little more than half contained, the Camp Fire is the deadliest and most destructive wildfire in California history and among the deadliest wildfires in modern U.S. history.

Over 10,000 (and counting) homes have been destroyed and 77 official civilian fatalities have been reported since the fire first broke out near the rural community of Pulga. Located just east of Chico, an entire town of 26,500 named Paradise has been virtually erased from the map by the inferno. More than 1,000 individuals in the Paradise area remain unaccounted for.

While climate change isn't the root cause of the fire, scientists have concurred that, much like other wildfires as of late, its overall impact has been intensified by climate change. This is a trend that will only worsen unless drastic steps to reduce greenhouse gas emissions and tread more delicately on the planet are taken.

Much needed wet weather is in the forecast for parched sections of Northern California later this week — most excellent news for residents of the Bay Area and beyond coping with dismal air quality as well as for the brave firefighters working around the clock to contain the fire. Gusty winds, however, are expected to arrive just ahead of the rain, the latter of which could result in localized flooding and mudslides.

Matt Hickman ( @mattyhick ) writes about design, architecture and the intersection between the natural world and the built environment.

Urban air pollution skyrockets as California wildfires burn
While the Camp Fire rages over 100 miles away in the Sierra Nevada foothills, San Francisco experiences an air pollution emergency.