Understanding Air Quality Alert Days

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September 12, 2019

During hot summer months, it’s not uncommon to hear air quality alerts announced over the radio or on local T.V. programs. But what do these alerts actually mean? What are the health risks? And how should you react when an air quality alert is issued for your area? We’ve outlined the basics.

What constitutes an Air Quality Alert?

The United States Environmental Protection Agency (EPA) has pollution sensors at over 1000 public locations throughout the country. These sensors monitor five different pollutants which are regulated by the Clean Air Act:

Each pollutant has a standard range, from good to hazardous, which is represented by the Air Quality Index (AQI). For most pollutants, AQI values below 100 indicate satisfactory air quality. As pollution levels rise above that threshold, the AQI scale is used to determine the relative health risk posed by higher measurements. Out of the five pollutants measured, ozone and particulate matter pose the greatest risk to human health and are most often the cause of air quality alerts.

Current AQI scores and individual pollutant readings are shared in real-time on the EPA’s “Air Now” database. Below is a breakdown of the EPA's AQI rating system:

AQI Level

Health Risk


1 - 50

Air quality is considered satisfactory, and air pollution poses little or no risk.


51 - 100

Air quality is acceptable, may be a moderate health concern for a small number of people who are unusually sensitive to air pollution.

Unhealthy for Sensitive Groups (USG)

101 - 150

General public is not likely to be affected. People with lung disease, older adults, and children are at a greater risk from ozone and particle pollution exposure. 


151 - 200

Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.

Very Unhealthy

AQI 201 - 300

Health alert: everyone may experience more serious health effects.


AQI 301 - 500

Health warnings of emergency conditions. The entire population is more likely to be affected.


Understanding AQI Readings

As the chart above shows, AQI alerts are color-coded and named. You may hear a newscasters announce an alert by color (for instance, a “Code Orange” alert) or the health level that the alert represents (such as “Unhealthy for Sensitive Groups” or “Very Unhealthy”). Other times, you may simply hear that there’s an “Action Day” in effect.

Action Days are the EPA’s term for any day when AQI scores rise above 101. Different states and environmental agencies may have their own term for unhealthy air days, like California’s “Spare the Air” alerts. Regardless of what term is used, the intention is to generate public awareness and encourage local residents to change their behavior on these days.

If you're unsure what an alert means for your health, start by looking up your zip code in the Air Now database to view the exact AQI value and pollutant responsible for the alert. Through this database, you can also sign up to receive email or text notifications when there's an air quality alert for your area and decode AQI readings using their AQI calculator

What's the Difference Between "Unhealthy" and "Unhealthy for Sensitive Populations?"

Very young children, older individuals, and people with preexisting conditions are considered sensitive groups that are more susceptible to the health impacts of air pollution, even at lower concentrations.

Babies lungs aren’t fully developed until around 37 weeks of age, and exposure to high levels of air pollution at this critical growth stage can increase their chances of developing chronic respiratory conditions like asthma. Young school-age children can also be considered part of the USG population because their lung tissue tends to be more sensitive and they generally spend more time running around outdoors. This combination makes them especially prone to short term irritation and infection.

Because PM2.5 pollution is so small, it can easily become imbedded in the respiratory tract and enter into the bloodstream. For elderly and sick individuals whose immune systems are less adept at fighting off pathogens, exposure to high O3 and PM2.5 levels can increase their risk of developing a respiratory infection, which can have ripple effects on their health. 


If you or a loved one falls into one of these categories, be aware of days the EPA identifies as “Unhealthy for Sensitive Groups” (USG), or “Code Orange” days. To protect your health, try to limit your outdoor time, avoid activities that might put more strain on your respiratory system, and take steps to protect your indoor air quality. When in doubt, consult your doctor about your unique risks and plan ahead for air quality emergencies.

Even for healthy populations, Unhealthy (Red) and Very Unhealthy (Purple) air days pose a significant health risk. For instance, on November 15, 2018, during the Paradise, CA wildfire, San Francisco, San Rafael, and Oakland all reached an AQI between 201 and 300 (very unhealthy). This surge in PM2.5 caused widespread eye, nose, and throat irritation and respiratory infections for local inhabitants.

On a long-term scale, routine exposure to high PM2.5 can add up to more serious lung problems. In recent decades, deaths from outdoor (ambient) and indoor air pollution have reached an annual total of around eight million — one million more than the annual death toll of cigarette smoking

How to Take Action

During air pollution alerts, one of the best ways to protect your health is to limit your outdoor time. Although you can’t single-handedly change outdoor conditions, you can take steps to keep your indoor air healthy. The average person spends around 90 percent of their time indoors, so the air quality in your home and office matters more than you might think. 

The biggest way that outdoor pollution infiltrates your home is through open windows and ventilation systems. Bathroom fans, kitchen fans, heating systems, air conditioning (HVAC) systems, air filters, and other ventilation devices that connect to the outdoors provide a direct avenue for pollution to enter into your home. Although these features help dissipate indoor pollution and are generally good for your indoor air quality, they can have the opposite effect when AQI alerts are in effect. When outdoor conditions are unhealthy, close your windows and vents and change your HVAC settings to recirculate indoor air rather than taking in fresh air from outdoors.

Without proper ventilation, it’s especially important to be mindful of your indoor activities, habits, and product choices. Pollution can be tracked into your home on your shoes and clothing, and can be generated from common indoor activities such as cooking (e.g. fuel combustion) and cleaning (e.g. chemical cleaning solutions). Nearly half of the 8 million annual deaths attributed to air pollution were the result of indoor conditions — such as household exposure to smoke from dirty cookstoves and cooking fuels.To keep your indoor air healthy when opening a window isn't an option, avoid pollution-generating activities and consider investing in air purifiers and fans to help circulate and filter your air.

Finally, don't forget to keep an eye on CO2 levels. Whenever you close outdoor windows and vents, indoor CO2 levels are likely to rise. If you start feeling lightheaded, dizzy, or are having difficulty concentrating, make sure to run a fan and open doors between rooms to help circulate air within your space.

Know Your Air

It’s easy to feel overwhelmed by air pollution alerts if you don’t have the tools to change your environment. If you live in an area where air quality alerts are common, don’t panic. Monitoring your indoor air quality at home and work is one way to take control of your health and the air you breathe.

Awair can help you understand exactly what’s in your air and provide you with the insight and guidance to improve your indoor air quality. To learn more about Awair and how it can help you take action when it matters most, follow the link below.


  • Ozone (03)
  • Particulate Matter (PM10 & PM2.5)
  • Carbon Monoxide (CO)
  • Sulfur Dioxide (SO2)
  • Nitrogen Dioxide (NO2)