EPA has proposed revisions to the nitrogen dioxide (NO2) air quality standard, the first adjustment since 1971. The proposed changes reflect the latest science on the health effects of exposure to NO2, which is formed by emissions from cars, trucks, buses, power plants, and industrial facilities and can lead to respiratory disease.
These proposed standards—which add a one-hour NO2 standard—and additional monitoring requirements will better protect public health by reducing people’s exposure to high, short-term concentrations of NO2, which generally occur near roadways, according to EPA. The proposal would also ensure that area-wide NO2 concentrations remain below levels that can cause public health problems.
Current scientific evidence links short-term NO2 exposures, ranging from 30 minutes to 24 hours, with increased respiratory effects, especially in people with asthma. These effects can lead to increased visits to emergency departments and hospital admissions for respiratory illnesses, particularly in at-risk populations such as children, the elderly, and asthmatics.
NO2 concentrations in vehicles and near major roads are appreciably higher than those measured at monitors in the current network. In-vehicle concentrations can be 2-3 times higher than those measured at nearby community-wide monitors. Near-road (within about 50 meters) concentrations of NO2 have been measured to be approximately 30 to 100% higher than concentrations away from major roads.
Individuals who spend time on or near major roads can experience short-term NO2 exposures considerably higher than measured by the current network, which are of particular concern for at-risk populations, including people with asthma, children, and the elderly.
EPA’s proposed revisions apply to the primary NO2 standard and would:
- Establish, for the first time, a one-hour NO2 standard at a level between 80 - 100 parts per billion (ppb). The Agency is taking comment on alternative levels for the 1-hour standard down to 65 ppb and up to 150 ppb.
- Retain the current annual average NO2 standard of 53 ppb.
- Add NO2 monitoring within 50 meters of major roads in cities with at least 350,000 residents; and
- Continue monitoring area-wide NO2 concentrations in cities with at least 1 million residents.
In addition to proposing an averaging time and a range of levels for the standard, EPA is also proposing a “form” for the new standard. The form is the air quality statistic that is compared to the level of the standard to determine if an area meets the standard.
For the new 1-hour NO2 standard, EPA is proposing that the form be a 3-year average of the 4th highest daily maximum 1-hour average concentration in a year, or a 3-year average of the 99th percentile of the annual distribution of daily maximum 1-hour average concentrations. (The 99th percentile for a year corresponds approximately to the 4th highest daily maximum.)
As an alternative to the proposed approach, EPA is requesting comment on supplementing the current annual standard with a community-wide 1-hour NO2 standard in the range of 50 – 75 ppb. Monitoring near major roads would not be required under this alternative.
The proposed changes would not affect the secondary NO2 standard; EPA is considering the need for changes to the secondary standard under a separate review.
EPA first set standards for NO2 in 1971, establishing both a primary standard to protect health and a secondary standard to protect the public welfare at 53 ppb, averaged annually. EPA has reviewed the standards twice since that time, but has chosen not to revise the standards at the conclusion of each review.
Under a judicial consent decree, EPA must complete this review of the primary NO2 standard by 22 January 2010. The current review focuses only on the primary NO2 standard. EPA will address the secondary standard for NO2 as part of a separate proposal in 2010.
EPA will accept public comments for 60 days after the proposal is published in the Federal Register. The agency will hold two public hearings in August 2009: one in Los Angeles and one in the Washington, D.C. area. EPA will provide details on the public hearings in a separate notice issued later this summer.
Annual average NO2 concentrations have decreased by more than 40% since 1980. All areas in the United States are well below the current (1971) NO2 standards with annual averages ranging from approximately 10 - 20 ppb.