The monitor colors on the map tell you how relatively clean or polluted the air is, and what this might mean for your health. The ranges are the corresponding Air Quality Index values.
Learn More . . . about Pollutants, Health Effects, and Standards; FAQ about the map
||0-50 = Good|
Air pollution poses little or no risk
||51-100 = Moderate|
Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people. Unusually sensitive people should consider reducing prolonged or heavy exertion outdoors.
||101-150 = Unhealthy for Sensitive Groups|
Members of sensitive groups may experience health effects. For example, people with lung disease are at greater risk from exposure to ozone, while people with either lung disease or heart disease are at greater risk from exposure to particle pollution. Active children and adults, and people with lung disease, such as asthma, should reduce prolonged or heavy exertion outdoors. The general public is not likely to be affected when the AQI is in this range.
||151-200 = Unhealthy|
Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects. Active children and adults, older adults, and people with lung disease, such as asthma, should avoid prolonged or heavy exertion outdoors. Everyone else, especially children, should reduce prolonged or heavy exertion outdoors.
||201-300 = Very Unhealthy|
Health alert: everyone may experience more serious health effects. Active children and adults, older adults, and people with heart or lung disease, such as asthma, should avoid all outdoor exertion. Everyone else, especially children, should avoid prolonged or heavy exertion outdoors.
||Good, close to 0|
Air pollution poses little or no risk. Grey is used instead of green for NO2, SO2, and CO to avoid confusion with ozone and PM2.5 on this map. Grey also may indicate that a PM2.5 or ozone monitor is measuring 0 or very close to 0 (this may be displayed as a negative value).
Health effects . . . learn more
Behind the Colors & Numbers
How we determine healthy vs. unhealthy air
The colors are based on the Air Quality Index (AQI). The AQI is calculated from the pollution level and the pollutant standards. Pollutants have different standards, but the AQI is a number that allows levels of different pollutants to be put on the same scale.
Think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 represents good air quality with little potential to affect public health.
An AQI value of 100 generally corresponds to the national air quality standard for the pollutant, which is the level US EPA has set to protect public health. AQI values below 100 are generally thought of as satisfactory. When AQI values are above 100, air quality is considered to be unhealthy at first for certain sensitive groups of people, then for everyone as AQI values get higher.
For example: an AQI of 100 for ozone corresponds to an ozone level of 0.070 parts per million (averaged over 8 hours). An AQI of 100 for PM2.5 (particles up to 2.5 micrometers in diameter) corresponds to a level of 35 micrograms per cubic meter (averaged over 24 hours).
Source: USEPA . . . learn more
What causes good or bad air?
Many factors can cause differences in the air quality across Massachusetts on any given day. The most influential factors are season, wind, weather and location.
How does the season impact air quality?Weather patterns most often associated with air pollution in Massachusetts are those that draw air up along the northeast corridor, through Washington D.C., Philadelphia, New York City, etc. before reaching Massachusetts. These patterns tend to occur more often, and last longer, in the summer.
Ozone is our primary seasonal pollutant because its formation depends on temperature and sunlight. The higher the temperature and the more direct the sunlight, the more ozone is produced. For this reason New Englands ozone season runs from April through September. During the cooler months of the year, lower temperatures and weaker sunlight keep ozone concentrations down.
This also is why ozone levels tend to follow a daily cycle, with lower ozone concentrations during the night and early morning, rising concentrations during the day, and falling concentrations after sunset.
Elevated fine particle pollution, however, can occur at any time of year.
Wind speed & direction why are they important ?Wind direction strongly influences air pollution levels. When winds blow across areas where there are many people, cars, factories, and power plants, the atmosphere becomes laden with pollutants, including fine particles, ozone, and chemicals that react to form ozone.
On hazy, hot, and humid summer days, winds in Massachusetts are usually blowing from the Southwest. These winds carry air pollution from the Ohio Valley and Mid-Atlantic states to Massachusetts. This process, known as regional transport, is responsible for a significant portion of the air pollution we experience in Massachusetts.
The left map below shows an example of ozone levels on a day when transport was from the southwest. The right map shows an example of where high ozone levels can develop when transport is from the west, particularly for the Cape and the Islands.
|Winds from the southwest bringing ozone and ozone-forming pollutants into Central MA from NYC/NJ area
||Winds from the west bringing ozone and ozone-forming pollutants to the Cape from NYC/NJ area|
When winds blow from less populated, less industrialized areas, much less ozone forms, no matter how sunny and hot it gets.
Meteorologists at the Massachusetts Department of Environmental Protection (MassDEP) get concerned when a certain summertime weather pattern appears. This pattern features a large high-pressure system over the Atlantic Ocean well south of New England that expands westward across the eastern United States. This "Bermuda high" pattern is responsible for some of our hottest and most humid weather - and for our worst ozone episodes.
Also, winds high above us can carry ozone and ozone-forming pollutants long distances. Not only does the poorly maintained car on the road in front of you foul the air you breathe, so do power plants hundreds of miles away.
The typical ozone episode occurs when a "pool" of ozone and ozone-forming pollutants builds over the industrialized midwest. That pool then flows eastward with the wind, passing through the heavily populated corridor stretching from Washington, D.C. to New York City. All that human activity generates more pollution and adds to the pool. When a broad southwesterly airflow develops over the eastern United States, it turns that pool northeastward toward Massachusetts. And when that happens in summer the weather is usually sunny and hot - perfect conditions for ozone production.
As for wind speed, a rule of thumb is, the stronger the wind, the faster pollutants can disperse and mix through a greater depth of atmosphere, cleansing the air. Fine particle concentrations typically increase at night into the morning hours when winds are light, then decrease as winds pick up in the daytime.
Weather Local weather can also affect how quickly pollutants move away from a local area. Normally, pollutants rise or blow away from their sources without building up to unsafe levels. But if the air gets trapped near the surface and the pollutants can't escape, then they may build up to unhealthy levels.
This trapping can happen when winds are slow or calm, and when warm air moves in over cold ground (called an inversion) keeping pollutants close to the surface. The pollutants are trapped near the surface because the cold surface air is heavier than the warm air above. Inversions often occur during colder months of the year resulting in a build up of fine particle pollution. Inversions also can occur on clear summer nights and can affect the next day's ground-level ozone concentrations.
An example of trapped particle pollution from an inversion is shown below.
Brown Cloud Polluted Day (January 21, 1999): This is a typical "brown cloud" polluted day in Boston. Note how the brown cloud appears to envelop the city but quickly thins out at higher elevations. These events tend to occur on calm winter mornings during rush hour traffic. Fine particulate matter (PM2.5) and black carbon (e.g., soot) levels may be high; ozone will be low; relative humidity (RH) may vary. Levels on this day were: PM2.5 = 40+ ug/m3 and RH = 69% (Source: CAMNET)
Location Local geography and location relative to pollution sources also play an important role in the differences in air quality across Massachusetts. Valleys can trap air pollution at the near surface level. For example, valley areas in western MA are prone to episodes of elevated fine particle concentrations. These occur when smoke from wood stoves gets trapped under inversions during the colder months of the year. In addition, locations downwind of air pollution sources will generally have higher levels of air pollution than locations upwind of the same sources.
Local sea breezes also can affect pollutant levels. They can often cause pollutant levels to drop when they draw cleaner air in from the ocean. But sometimes the air offshore is dirtier, and in those cases pollutant levels can increase. And because a sea breeze is a closed air current, it can recirculate polluted air and sometimes worsen air quality.