Air Quality


Air quality refers to the degree to which the ambient or outdoor air in our surrounding environment is free of pollution. It is typically measured near ground level, away from direct sources of pollution. The National Ambient Air Quality Standards (NAAQS) has established standards or limits for six air pollutants, known as the criteria air pollutants, including carbon monoxide, lead, nitrogen oxides, ground-level ozone, particle pollution (often referred to as particulate matter), and sulfur oxides. In NH, Ozone (O3) and fine particulate matter (PM2.5) are the primary pollutants of concern.  In New Hampshire (NH), there are 14 air quality monitoring stations monitoring these pollutants. 

The U.S. Environmental Protection Agency's (EPA) Air Quality Index (AQI) is a tool to help you quickly learn when air pollution is likely to reach unhealthy levels. Local TV stations, radio programs, and newspapers carry these air quality forecasts to tell you when particle levels are likely to be unhealthy. When particle pollution levels are high, individuals are encouraged to do easier outdoor activities (walking instead of running or using a riding lawn mower instead of a push mower), exercise away from roads and highways because particle pollution is usually worse in these areas, and reduce the overall amount of time spent outside.

Air Quality Data Overview

Check out the points below for the main takeaways from this page.

• In Hillsborough County in the past year, the majority of days have been considered good air quality days, while some have been considered moderate air quality days (3%). The number of moderate air quality days has decreased from 2015 to 2020 in Hillsborough County. 

• Ozone is the main outdoor air pollutant in Nashua and New Hampshire, followed by particulate matter 2.5. In Hillsborough County, an estimated 97% of emitted Ozone and 80% of emitted PM2.5 is from transportation. 

• In 2020, Hillsborough County did not surpass the national air quality standard levels for Ozone, PM2.5, NO2, SO2, or CO. 

*** EPA Annual statistics for 2020 are not final until May 1, 2021

Outdoor Air Quality

Outdoor air consists of the the air outside of buildings, including from the ground level to several miles above the Earth's surface. Outdoor air provides gases that are essential in sustaining life and protecting Earth from harmful radiation, making it a valuable resource for current and future generations. The quality of outdoor air is a public health priority because pollution can compromise the health and safety of humans and the environment. People at greater risk for health effects from air pollution include those with respiratory diseases (such as asthma), children, and older adults. 

Since the 1990s, outdoor air quality has improved, but many challenges remain in protecting communities from air pollution. In the United States (U.S.), ground-level ozone and particle pollution are just two of the many threats to quality of our air and the health of our community.

Air Quality Index

The AirData Air Quality Index (AQI) Summary Report is an annual summary of Air Quality Index (AQI) values for counties compiled by the EPA. The Air Quality Index is a good indicator of overall air quality, and is a measurement from 0-500 that takes into account all air pollutants. The higher the index value, the greater the level of air pollution in that community. The below graphs  show the Air Quality Index max value of the year as well as the number of days with good, moderate, unhealthy, and very unhealthy AQI days. 

# Days Good

Number of days in the year having an AQI value 0 through 50.

# Days Moderate

Number of days in the year having and AQI value 51 through 100.

# Days Unhealthy for Sensitive Groups

Number of days in the year having an AQI value 101 through 150.

# Days Unhealthy

Number of days in the year having an AQI value 151 through 200.

# Days Very Unhealthy

Number of days in the year having an AQI value 201 or higher. This includes the AQI categories very unhealthy and hazardous. Very few locations (about 0.3% of counties) have any days in the very unhealthy or hazardous categories.

Outdoor Air Pollutants in NH and Greater Nashua

Air Quality Statistics

The Air Quality Statistics Report, published by the EPA, displays local air pollution values in comparison to national standards. These statistics show if Hillsborough County's maximum air quality statistics are above the level of the national standards, per year. The values shown are the highest reported during the year by all monitoring sites within the Hillsborough County catchment area. The five pertinent air pollutants show below are Ozone, PM2.5, Carbon Monoxide, Sulfur Dioxide, and Nitrous Oxide. 


Ozone (03) is a colorless to blue gas with a pungent odor. It is created when nitrogen oxides and volatile organic compounds mix and are exposed to heat and sunlight. Everyone's exposure to O3 is different because it depends on where you live and work and the amount of time you spend outdoors.

O3 is a public health issue because it effects the health of everyone. Exposure can lead to headaches, coughing, dry throat, shortness of breath, a heavy chest, and fluid in the lungs. Higher levels of exposure can lead to more serious symptoms, with chronic exposure sometimes leading to asthma. Although symptoms are mild for most people, those at high risk and people who are active outdoors can develop severe illness when O3 levels are elevated (CDC). 

National air quality standard for Ozone

0.12 ppm (1-hour), 0.070 ppm (8-hour)

O3 1-hr 2nd Max

For Ozone, the 2nd highest daily max 1-hour measurement in the year.

O3 8-hr 4th Max

For Ozone, the 4th highest daily max 8-hour average in the year.

Particulate Matter (PM2.5)

Particulate matter (PM) consists of particles of dust, dirt, soot and smoke, and little drops of liquid in the air. While some particles, such as soot or smoke, are large or dark enough to be seen, others are so small that they cannot be seen by the naked eye (EPA). Small particles are a public health concern because they are most likely to cause health problems.

Exposure to particulate matter can result in adverse birth outcomes (such as low birth weight), severe asthma symptoms, breathing problems, decreased lung growth in children, early death, increased emergency department visits and hospital stays for breathing and heart problems, and lung cancer. 

National Air Quality Standard for PM2.5

35 ug/m3 (24-hour), 12.0 ug/m3 (annual)

PM2.5 Wtd Mean

For PM2.5, the Weighted Annual Mean (mean weighted by calendar quarter) for the year.

Ozone & PM2.5

Ozone and PM2.5 are the primary outdoor air pollutants of concern in both Nashua and NH. When nitrous oxide and volatile organic compounds are released into the air via combustion processes, such as transportation or industry, these pollutants react with sunlight to create ozone. Because of this reaction between air pollutants and sunlight, ozone is most likely to be at unhealthy levels during hot, summer days in urban environments. Ozone is the main component of what people refer to as "smog".

PM2.5 is also primarily emitted by combustion processes.

Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless, and toxic gas. It is produced by incomplete combustion of carbon-containing materials occurring when insufficient oxygen is used in the fuel burning process. Examples of carbon monoxide production include vehicle exhausts, fuel burning furnaces, coal burning power plants, small gasoline engines, portable gasoline-powered generators, power washers, fire places, charcoal grills, marine engines, forklifts, propane-powered heaters, gas water heaters, and kerosene heaters.

Each year, more than 400 people in the U.S. die from unintentional carbon monoxide poisoning (CDC). Exposure to carbon monoxide is a health concern as it inhibits the blood’s ability to send oxygen to body tissues and vital organs. Common symptoms of carbon monoxide exposure include headache, nausea, rapid breathing, weakness, exhaustion, dizziness, and confusion. Although carbon monoxide exposure does not discriminate, infants, older adults, and people with chronic conditions are more prone to illness or death due to carbon monoxide poisoning. To reduce carbon monoxide pollution in outdoor air, the EPA regulates air quality standards.

National air quality standard for CO

35 ppm (1-hour), 9 ppm (8-hour)

CO 1-hr 2nd Max

For Carbon Monoxide, the 2nd highest 1-hour measurement in the year.

CO 8-hr 2nd Max

For Carbon Monoxide, the 2nd highest non-overlapping 8-hour average in the year.

Nitrogen Dioxide

Nitrogen dioxide (NO2) is a highly reactive gas released into the air we breathe when fuel is burned from motor vehicles, power plants, and off-road equipment. Exposure to a high concentration, even for a short period of time, can irritate the respiratory system, causing coughing, wheezing, or difficulty breathing (EPA). People at greater risk for health effects from nitrogen dioxide include people with respiratory diseases (such as asthma), children, and older adults. When high risk individuals are exposed to nitrogen dioxide, they have the potential to experience severe respiratory symptoms that can result in hospital admissions and emergency room visits. Prolonged exposures can contribute to the development of asthma and even increase susceptibility to some respiratory infections. 

Sulfur Dioxide

Sulfur dioxide (SO2) is a colorless gas with an irritating, pungent odor. Sulfur dioxide is primarily released into outdoor air from fossil fuel combustion at power plants and other industrial facilities. Smaller emissions of sulfur dioxide can result from industrial processes and vehicles or heavy equipment that burn fuel containing a high sulfur content.    

Sulfur dioxide is common as it is used in many industries to manufacture sulfuric acid, paper, and food preservatives. Use of sulfur dioxide in the workplace puts many workers at risk for exposure. Breathing in air contaminated with sulfur dioxide can cause irritation to the eyes, nose, and throat with symptoms including nasal mucus, choking, cough, and reflex bronchi constriction (CDC).

National air quality standard for SO2

75 ppb (1-hour), 140 ppb (24-hour), 30 ppb (annual)

SO2 24-hr 2nd Max

For Sulfur Dioxide, the 2nd highest 24-hour average measurement in the year.

SO2 Annual Mean (1-hr)

For Sulfur Dioxide, the annual mean of all the 1-hour measurements in the year.

Indoor Air Quality

On average, Americans spend approximately 90% of their time indoors.

Concentrations of some pollutants are often two to five times higher in indoor settings than outdoor. The quality of air within and around buildings and structures can impact the health and comfort of its occupants. Identifying and controlling common air pollutants found indoors can reduce risk of health concerns. Health effects caused by indoor air pollutants are not always immediate, as they can also be experienced years later.

Indoor environments are complex, causing occupants to have possible exposure to many contaminants in public spaces. Contaminants in the form of gases and particles can be produced from office machines, cleaning products, construction activities, carpets, furnisher, perfumes, cigarette smoke, water-damaged building materials, microbial growth (fungal, mold, and bacterial), insects, and outdoor pollutants. Indoor temperatures, humidity, and ventilation levels can also affect the indoor air quality (EPA).

Appropriate building ventilation, proper circulation of air throughout a building, is a critical component of indoor air quality. The ventilation or the heating, ventilating, and air-conditioning (HVAC) system of a building supplies and removes air naturally to and from a space. Identifying and controlling common air pollutants indoors to reduce risk of health concerns is a public health priority for our community. Health effects caused by indoor air pollutants do not always present immediately, as they can also be experienced years later. Infants and young children, older adults, and people with cardiovascular or respiratory disease are at greater risk for health effects from indoor air pollutants, especially since they tend to spend even more time indoors.

For more information on Indoor Air Quality, visit our Healthy Homes page!

Health Equity & Air Quality

A person's health can be seriously impacted by their race, ethnicity, gender, income level, education, and other socioeconomic factors. In regards to air quality...

• Researchers have found greater risk for African Americans from hazardous air pollutants, including pollutants that come from traffic. Due to decades of residential segregation, African Americans tend to live where there is greater exposure to air pollution. Those who live in predominantly Black communities suffer greater risk of premature death from particle pollution than those who live in communities that are predominately white (American Lung Association). 

• Non-Hispanic blacks and Hispanics were more likely to live in counties that had worse problems with particle pollution, researchers found in a 2011 analysis. Non-Hispanic blacks were also more likely to live in counties with worst ozone pollution (American Lung Association).

• Unemployed people, those with low income or low education, and non-Hispanic blacks were found to be more likely to live in areas with higher exposures to particle pollution in a 2012 study (American Lung Association).

• Low socioeconomic status consistently increased the risk of premature death from fine particle pollution among lower income people studied in the largest study of particle pollution-related mortality nationwide (American Lung Association).

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