Is the Air Quality Index enough? What the AQI can — and can't — tell you about how healthy the atmosphere is.

After fires destroyed multiple neighborhoods across Los Angeles County, many residents looked up to find blue, smoke-free skies this week. The Air Quality Index (AQI) — which measures the concentration of pollutants in the air — seemed to confirm all was well in the L.A. atmosphere, with low numbers that suggested clean, healthy air.

But many Angelenos aren’t sure if the AQI tells the full story, especially given how their bodies are seemingly reacting to the outdoor conditions. A Frogtown resident, who lives miles away from each of the fires, says her asthma is still flaring up, even when wearing a mask outside and using an at-home air purifier — despite the positive AQI score in her neighborhood. A Los Feliz resident tells Yahoo Life that in his area, the AQI has been indicating even better air quality than before the fires, which is “hard to believe,” he says. “Despite the good to moderate readings, I’m still feeling a burn in my nostrils and dryness in my throat.” Is the AQI really a good indicator of the quality of the air? Experts say there’s a good reason to be cautious. Here’s what to know.

What is the Air Quality Index?

The Air Quality Index is a tool used by the Environmental Protection Agency (EPA) in order to communicate outdoor air quality. It’s a scale of 0 to 500, with the higher number indicating the air is potentially more dangerous to one’s health. An AQI value of 50 or below represents good air quality, per the EPA, while an AQI value over 300 represents hazardous air quality.

An AQI value of 100 is considered the threshold for safe air quality. Values at or below 50 are considered good, with 51 to 100 considered “moderate,” or potentially risky for people who are “unusually sensitive to air pollution.” When the AQI is above 100, the air is considered unhealthy, first affecting sensitive groups, like children, the elderly and people with respiratory conditions, and then everyone as the number increases.

Here is the Air Quality Index chart from the Centers for Disease Control and Prevention:

Lung-Wen Antony Chen, an associate professor in the Department of Environmental and Occupational Health at the University of Nevada, Las Vegas School of Public Health, tells Yahoo Life that the AQI is “based on the measurements of criteria air pollutants, known as CAPs, such as PM2.5 and ozone.”

PM2.5, according to the EPA, refers to fine particulate matter that measures 2.5 micrometers or smaller in diameter, which makes it small enough to penetrate the lungs and even enter the bloodstream. Ozone, meanwhile, is a gas that forms when pollutants react in sunlight. It can irritate the lungs, inflame and damage airways and aggravate diseases such as asthma, emphysema and chronic bronchitis.

According to Chen, “while high AQI would certainly indicate an unsafe condition, a low AQI does not guarantee healthy air quality,” he says.

Amy MacPherson, a spokesperson for the California Air Resources Board (CARB), the lead agency for climate change programs and oversees all air pollution control efforts in the state, tells Yahoo Life that “the reliability of the AQI reading depends on the accuracy of the air monitors.” While some AQI readings are based on low-cost sensor data, others are based on regulatory-grade air monitors. “The regulatory-grade air monitors used to calculate AQI are extensively calibrated and maintained to rigorous standards,” she says. “CARB is confident that these AQI readings are accurate.”

What doesn’t AQI tell us?

Chen says the reason AQI does not tell the whole air quality story is because it does not include hazardous air pollutants (HAPs). “These are chemicals usually in lower concentrations but more toxic in nature,” he explains, noting that the EPA has a separate HAPs monitoring program.

Additionally, he says, AQI cannot tell us the exposure of air pollutants through ingestion when these pollutants fall out onto water and food, for example. Instead, “specific toxic air measurements are usually conducted on demand when there is a concern.”

Shahir Masri, an assistant specialist in air pollution exposure assessment and epidemiology at the UC Irvine Program in Public Health, tells Yahoo Life that the AQI can’t tell us about very localized air pollution. Is there a small factory near your house? That may cause elevated exposures in your neighborhood, for instance, which cannot be captured by the regional EPA monitor.

“Similarly, the AQI considers average air quality over specified time windows,” he explains. “For PM2.5 specifically, it considers a 24-hour average. Thus, during a scenario like a major, short-lived wildfire, individual residents may experience higher PM2.5 exposures than that which may be reflected by the AQI, particularly given that smoke plumes can shift — sometimes hitting the EPA monitor and sometimes blowing in the opposite direction.” In these cases, Masri points to the AQI regional map, which can reveal a “bigger picture of what neighboring areas are looking like.”

MacPherson notes that CARB has an air toxics monitoring program and “works closely with the South Coast Air Quality Management District, which owns and operates toxics monitors in the Los Angeles area.”

“The methodology for collecting, analyzing and reporting air toxics data is also complicated and time-intensive, meaning it takes days or weeks to report air toxics data as compared to minutes or hours for particulate matter data,” she says.

How can we properly assess air quality?

Chen says that for outdoor air quality, the best resources are still from the EPA air monitoring stations nearby. “Most of the stations measure criteria for air pollutants, but some contain more specific measurements,” he explains.

Masri says that while the AQI is still a “good resource,” there are other options for people at home to check the quality of their air, noting that “recent technological innovation has led to the development and deployment of thousands of low-cost air pollution sensors.” These sensors, he says, are low-cost, mobile, affordable and easy to maintain, making them an option for people concerned with their air quality.

“In 2017, the PurpleAir company began to deploy low-cost air quality sensors which now provide real time PM2.5 data throughout the U.S. and abroad,” Masri says, noting that you can purchase these devices on the company’s website, where they go for around $209. Meanwhile, he says, the Atmotube company also sells small mobile sensors at a similar cost — about $179.

“Both devices have been validated for their accuracy in monitoring PM2.5,” he says.

In the case of PurpleAir, Masri notes that people can “tap into the national data without even having to own a sensor device.” You can look at the PurpleAir Map online to see air quality around the country.

What can I do to protect myself from bad air?

MacPherson says that people are impacted by air pollution differently and that if you are experiencing health concerns or having trouble breathing, it is best to call your medical provider for advice. “Some groups are more impacted by wildfire smoke — especially children, seniors, pregnant people, people with asthma or other respiratory conditions and people suffering from heart conditions,” MacPherson notes. “People in these sensitive groups should take extra precautions to protect their health.”

If you are concerned about your air quality outside, there are steps you can take to protect yourself and loved ones:

• Stay inside when you can: Stay indoors during periods of poor air quality, especially if you are in sensitive groups like children, the elderly or those with respiratory conditions like COPD or asthma.

• Shut windows and doors: Prevent polluted air from entering your home by sealing windows and doors. If you live in a home with windows that don’t seal all the way, you can use caulk or weatherstripping to close gaps.

• Use air purifiers: Go for one with a HEPA filter to reduce indoor pollution.

• Skip the leaf blowers: Air blowers, like leaf blowers, stir up dust, pollen and unhealthy particles, which can be especially dangerous during poor air quality days.

• Wear a mask: If you must go outside, wear a high-quality mask like an N95 to protect your lungs.

• Keep your vents clear: Regularly clean air vents can help you reduce dust, allergens and any dangerous particles from the outside.