The Santa Ana winds caused a massive wildfire this month in Ventura County, with great photos of orange sky in Oxnard reminiscent of 2020 in the San Francisco Bay Area. Even if Mountain fire is now almost contained (as is the Sandy fire in Monterey County), it was the most destructive fire to hit Southern California in six years, polluting air past Ventura towards Santa Barbara and beyond. With Santa Ana winds and an increasingly long fire season, smoke may spread again in November and into December in southern and northern California, as happened in 2017 and 2018.

And it’s not just a problem on the west coast: the Northeast is facing its own worst fire season in more than a decade, call evacuation in New York over the weekend.

In the affected areas, residents must immediately protect themselves from inhaling smoke from forest fires, which in composition is comparable to passive smoking without nicotine. Hazardous smoke particles can cause or exacerbate heart and lung disease miles away.

South Coast Air Quality Management District recommends stay indoors and run an air conditioner or air purifier. This general advice is useful but does not go far enough. There are five specific steps that can safely limit your exposure to extreme levels of toxic particles during these large fires.

First, be informed. Local air quality monitors, PurpleAir’s network of researchers and other online services can provide a clear picture of particulate pollution in your area. Handheld and stationary personal particle monitors are also available for purchase. You can’t trust the color of the sky, which can be deceiving: In 2020, wildfires turned San Francisco’s sky dark orange, but the air quality was actually worse after the sun returned and the sky looked more normal, because then the smoke particles fell closer to the ground, up into the air people breathe.

Second, stay indoors and keep your windows and doors closed as much as possible reduce exposure to outdoor pollutants with as much as half.

But particles still seep through cracks, and closing windows and doors also traps exhaled air, increasing the risk of spreading respiratory diseases like covid-19 or the flu. That leads to the third important step: Clean up the outdoor pollutants that come indoors, as well as potentially dangerous breath particles.

Central ventilation, even if filtered, usually needs to be supplemented with suitably sized room air cleaners. In 2023, the Centers for Disease Control and Prevention recommended at least five air changes per hour for occupied indoor spaces, and this month California Department of Public Health reiterated this recommendation specifically for classroom.

The number of air cleaners needed to achieve this goal varies depending on the dimensions of the room, the model of air cleaner, and the speed and noise level that occupants find acceptable for the air cleaner. High efficiency particulate air or HEPA cleaners run at five to 10 air changes per hour cost $1 to $5 per square foot for typical ceiling heights, and indoor particulate exposure can be reduced by 10 to 50 times relative to outdoors, depending on the degree of particles still seeping into the room when doors and windows are closed.

If getting cleaners for your entire home is daunting, you can focus on making at least one room safe, such as your bedroom. Low-cost (about $20) infrared motion switches connected to purifiers can save power and extend filter life by turning on air purifiers when people enter the room and turning off with a time delay after they leave.

HEPA purifiers often sell out during peak forest fire season. A do-it-yourself cleaner can be entered 10 minutes with more consistently available components, a box fan and a MERV 13+ furnace filter sold online, at typically five to 10 times the cost cheaper than HEPA alternative. Alternatively, even if lower quality filters are the only ones in stock in stores or online, some filtering is much better than nothing.

Fourth, in addition to indoor safety, if you need to drive in smoky conditions, you can change the car’s air system to recirculate. As a long-term solution, in many cars you can also upgrade your cabin air filter to a HEPA model.

Finally, whether you’re outdoors or indoors without an air purifier, carry one N95, N99 or reusable elastomer mask like a P100, all of which are designed to reduce particulate exposure from wildfire smoke by at least 20 times as long as they are fresh and fit well. If these are difficult to wear, especially in hot weather, vented industrial helmets with battery-powered fans and filters can be used more comfortably (although they tend to be expensive, starting at around $450).

As climate change intensifies, wildfires are likely to become more frequent and more severe. Take these precautions for fire smoke as well protects against airborne threats such as respiratory diseases and pollution in general. Don’t wait until the sky turns orange to take action.

Devabhaktuni Srikrishna is an electrical engineer, founder of the air quality website www.patientknowhow.com and vice Chairman of Control and Limitation for the American Assn. of Aerosol Research Annual Conference.