Air Quality: Why It Suffers in Summer

A smoky day of a city
COPYRIGHT, Jong-Won Heo / Getty Images

For summer lovers, the hotter the air temperature, the better. But hot doesn't always mean healthy. Besides putting your body at increased risk for heat illness, the summer sun can actually increase your exposure to air pollution and poor air quality.   

High Pressure Brings Stagnant Air

High pressure systems are generally associated with fair weather, but in summer they can cause heat waves and stagnant air. To understand how, let's look at how high pressure systems work.

Highs exist wherever there's a build up of air molecules (air pressure) at one location compared to surrounding locations. Because they have more air, and because air always moves from areas of high to low pressure, they constantly push air away from their centers into areas of lower pressure. This leads to diverging winds (winds that spread out) at the surface. As air near the surface spreads away from the high center, air from above it sinks down toward the surface to replace it. This sinking air creates an invisible boundary around the high pressure area. Anything within this boundary becomes "grounded" and trapped within it, including hot air. (This is why your weatherman refers to it as a "dome" of high pressure.) 

And why is this dome significant? Well, just like if you took a lid and placed it upside-down onto a table, creating a barrier , the sinking air in a high pressure system traps air near the ground. High pressure creates a stable atmosphere, and while you'd think stability would be a good thing, in summer it means you get stagnant, still air. Without being able to flow freely and mix with air in the upper atmosphere, this trapped air near the surface pins dirt, smoke, and emissions from cars, trains, and power plants near the surface where they accumulate -- and where we breathe them in.

Sunlight Produces Ground-Level Ozone

The sun, the very symbol of summer, is another cause of unhealthy air in the form of ozone pollution.

Ozone forms when incoming ultraviolet radiation (sunlight) chemically interacts with nitrogen dioxide (NO2), which is present in the air largely as a result of the burning of fossil fuels, and breaks it apart into nitric oxide and an oxygen atom (NO + O). This single oxygen atom then combines with an oxygen molecule (O2) to produce ozone (O3). Summer's longer days and more plentiful sunshine mean 

How will you know when unhealthy levels of ozone or other pollutants bombard the air? Why, by checking your air quality index!

The Air Quality Index (AQI)

Maintained by the Environmental Protection Agency, the air quality index (AQI) is an index for reporting daily air quality. It tells you how clean or polluted your local air is, and how likely it is to affect your health in the hours and days after breathing it in. (Of the 5 major air pollutants monitored by the AQI (ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide) ground-level ozone and airborne particles are the most dangerous to humans.) 

The AQI is divided into six categories ranging from good to extremely hazardous. Similar to pollen index forecasts, each AQI category is color-coded so that people can understand at a glance whether air pollution reaches unhealthy levels in their community.

The AQI is divided into six categories as follows: 

Color Air Quality Conditions Health Concern Levels & Meanings AQI Values
Green Good Little or no risk. 0-50
Yellow Moderate People with a sensitivity to certain pollutants may have respiratory problems. 51-100
Orange Unhealthy for sensitive groups People with heart or lung disease may be affected. 101-150
Red Unhealthy General public may experience adverse effects; Sensitive groups, more serious effects. 151-200
Purple Very unhealthy General public should be alert and may experience serious health effects. 201-300
Maroon Hazardous Pollution levels have reached dangerous levels; general public may experience serious effects. 301-500

Whenever the AQI reaches the unhealthy, or orange level, it is said to be an "action day." This means you should take care to reduce exposure to the pollution by reducing time spent outdoors.  

To check your local AQI, visit and enter your zip code in the banner at the top of the homepage.

Resources & Links:

"Chemistry in the Sunlight." NASA Earth Observatory