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New Hampshire Department of Environmental Services

Ozone Levels in New Hampshire

Where are they unhealthy, and why?
Ground-level ozone in the ambient (outdoor) air has been measured at various sites in New Hampshire from May through September since 1970. Measurements vary considerably from site to site, but levels exceeding the federal health-based standards have been recorded in recent years, and the numbers of days per year when this occurs are shown in the graph below.

NH Ozone Exceedence Days

Each year, NHDES reviews its monitoring data for comparison to the National Ambient Air Quality Standards (NAAQS). In general, values consider the three most recent years for an averaging period. Annual values are presented in the charts below. Note that the highest monitored concentration for the area was used in each year.

ozone trends southern and southwestrn nh
ozone trends northern and western nh

Though the overall trend is downward, portions of New Hampshire are classified as nonattainment areas under the current eight-hour standard, based on the number of unhealthy days (or exceedances) measured by the air monitoring network A map showing New Hampshire’s 8-hr nonattainment areas is available at EPA’s web site.

Elevated ozone levels in New Hampshire are caused by the transport of ozone and ozone precursors into the state from upwind areas in the Northeast and industrial Midwest (i.e., areas to the south and southwest of New Hampshire). Emissions of NOx and VOCs from sources in these upwind areas react photochemically to form a large ozone plume, which is transported to New England by the prevailing winds during ozone episodes and other warm summer days.

During a typical summer, an ozone smog event strikes the state about one out of every four to five days. Hardest hit is the southeastern portion of the state. The worst ozone days are created when the wind flows over the east coast corridor cities of Washington, Baltimore, Philadelphia, and New York prior to reaching New Hampshire. The seacoast often gets the worst pollution during an episode when a sea breeze brings Boston-produced ozone pollution in from the ocean to mix with ozone pollution blown-in from other areas.

Note: The current National Ambient Air Quality Standard for Ozone is an 8-hour average concentration of 0.075 ppm. On June 15, 2005 the 1-hour ozone standard was revoked. See for more information about the 1-hour ozone standard.

NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

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