Air quality in Baltimore

What is the general air quality in Baltimore?

Baltimore air quality meets federal attainment levels for all regulated criteria pollutants except for ozone. Ozone has long posed challenges for the city. Since at least 1996, Baltimore has exceeded the federal limit on unhealthy ozone days.¹ Most recently, in the period from 2016 to 2018, the city experienced a weighted average of 14.2 unhealthy ozone days, several times above the federal limit of 3.2. The ‘serious nonattainment’ status has landed Baltimore an “F” rating from the American Lung Association’s State of the Air report. Out of 229 metropolitan areas included in the report, the greater Washington-Baltimore-Arlington area ranked 20^th for worst ozone.

Fine particulate matter, or PM2.5, is another pollutant of concern in the city. While Baltimore has met federal levels for 24-hour and annual PM2.5 since 2008, 2019 saw an uptick of particulate pollution. Similar increases in PM2.5 and ozone in 2019 (and often since 2017) have been observed in numerous cities across the US. Denver, Dallas, Chicago, and Houston are a few examples of cities that follow similar trends of recently increased pollution levels. Though they may not represent a sustained trend, these pollution increases are concerning. They may owe to the Trump administration's relaxed EPA regulations and enforcement of polluters. Adding to this problem during the COVID-19 pandemic, power plants and factories have been allowed to self-report emissions without any EPA oversight.² This change is estimated to have contributed to a 13 percent increase in pollution from counties with more than six facilities that are normally required to comply with routine reporting.

In 2019, Baltimore annual PM2.5 levels were 11.2 μg/m³, just barely meeting the federal annual average threshold of 12 μg/m³ though failing to meet the more stringent World Health Organization target for annual PM2.5 of 10 μg/m³.

February, July, August, November, and December of 2019 all experienced PM2.5 levels categorized as US AQI “moderate.” In previous years, such as in 2018 and 2017, no months fell into this higher AQI category. Increased pollution in the winter is often attributed to increased emissions as a result of home and building heating as well as due to a pollution “trapping” weather effect known as a temperature inversion. Baltimore elevated pollution levels in the summer, on the other hand, are often attributed to heightened ozone levels that increase with rising temperatures and more direct sunlight.

Due to the fast-changing, dynamic nature of air quality, real-time data is the best resource for understanding local pollution and the risk to health. Follow Baltimore forecast air quality data at the top of this page and on the IQAir app to plan ahead and take action to reduce your pollution exposure.

Has air quality improved in Baltimore?

Long-term trends reveal greatly improved air quality in Baltimore since at least 1996. These trends reveal pollution mitigation efforts by Maryland and the federal government – namely, increasingly stringent regulations on industry and vehicles as well as new technologies for more efficient energy.

Fine particulate pollution, or PM2.5, has met federal attainment levels for 24 hour and annual levels since 2010. Moreover, PM2.5 levels have shown consistent year-over-year improvements, with exception to 2019.

Ozone levels have wavered but also improved slightly. Still, Baltimore has never met federal attainment levels for ozone pollution. The monitoring period from 2013 to 2015 was the closest Baltimore ever came to meeting attainment levels, but ozone levels were still more than two times the federal target. Since 2015, ozone levels have risen. The monitoring period of 2016 to 2018 experienced nearly double the number of unhealthy days as in the period from 2013 to 2015. Baltimore air quality still has a ways to go with regards to ozone pollution. Transitioning more residents to electric vehicles or greener transport, investing in renewable energies, implementing more stringent emission standards on industrial facilities, and working with neighboring upwind states to reduce transboundary emissions all offer opportunities for Baltimore to further reduce ozone levels in order to meet federal attainment.

What causes the most air pollution in Baltimore?

Baltimore air pollution levels result from city sources, including building heating, traffic, and industry as well as out-of-city and out-of-state sources carried over by wind.

An estimate from the Maryland Department of the Environment (MDE) found that roughly two-thirds, or 70 percent, of Baltimore smog may be attributable to upwind states.³ Namely, Baltimore’s ozone problem is largely the result of power plant and vehicle emissions carried over from Pennsylvania, West Virginia, Indiana, Ohio, and Kentucky.

While many of these states have regulations roughly in line with Maryland’s, enforcement isn’t as rigorous, resulting in over polluting. The MDE identified 19 over-polluting power plants in the five-state area and petitioned the US Environmental Protection Agency (EPA) in November 2016 that they be required to turn on pollution controls in general compliance with, or to the precedent of, Maryland law. If these 19 plants were to use their pollution controls effectively, the region could be spared from 39,000 tons of NOx. After a lengthy legal battle, the EPA denied its role in protecting Maryland from transboundary air pollution. The MDE continues its legal fight to hold these out of state polluters accountable.

Use the Baltimore air pollution map to discover periods of transboundary pollution flowing from upwind states as well as the movement of emissions from local pollution sources.

When does Baltimore’s have the worst air quality?

Baltimore air quality index (AQI) levels are highest over the winter months. AQI ratings provide a snapshot of pollution levels across six key criteria pollutants. High AQI levels represent the riskiest air quality overall, which in Baltimore tends to occur in November, December, and January.

High winter AQI levels result from spikes in fine particulate matter or PM2.5 as a result of increased wood burning, car idling, and building heating. Weather events known as cool air inversion events in which cold surface-level air becomes trapped and unable to disperse due to a warmer air layer above, traps cold air and causes pollution emissions to accumulate, creating low-hanging smog.

In the summer, heightened pollution levels are often attributed to increases in ozone pollution. Ozone is created when sunlight and heat trigger chemical reactions between precursor pollutants in the air. The hotter the environment, the more ozone is produced. Ozone pollution tends to be the worst during daylight hours and on weekdays when traffic is relatively heavy.

How does Baltimore's air quality compare to Washington DC?

Despite a mere 40 miles of separation and comparable populations, Baltimore air quality is less healthy than Washington DC air quality, particularly for ozone pollution.

While Washington DC is close to meeting federal attainment levels for ozone with a weighted average of 5.2 unhealthy ozone days from 2016 to 2018, Baltimore experienced a weighted average of 14.2 unhealthy days for the same time period.

Washington DC air quality likely fares better than Baltimore's as a result of less industry and a greater share of electric vehicles on the road.⁴ Such a contrast provides a possible roadmap for Baltimore to obtain air quality compliance.

+ Article Resources

[1] American Lung Association. (2019). State of the air – 2019.
[2] Kowalski K. (2020, July 30). Wheeler dismisses study claiming EPA role in elevated air pollution, COVID-19 cases.
[3] Chesapeake Bay Foundation. (2020). Stopping upwind air pollution.
[4] Atlas Public Policy. (2019). State EV registration data.