CO Attainment Status and Trends

The Clean Air Act defines an area as attainment or nonattainment based on a “design value” (see Criteria Pollutants). Please read below for information about carbon monoxide (CO) attainment status and design value trends.

New Hampshire’s Attainment Status
The eight-hour CO design values did exceed the NAAQS in the late 1970s through the 1980s, leading to the cities of Nashua and Manchester designated as nonattainment areas; these areas were formally re-designated attainment in 2001. However, maintenance provisions, in place to ensure air quality continues to meet the standard, still apply in the areas formally designated as nonattainment. Please see NHDES’ State Implementation Plans page to learn more and find related reports for CO.

Historical Trends
Concentrations of CO have improved significantly since 1976 when NHDES began monitoring this pollutant. The charts below compare New Hampshire’s maximum one-hour and eight-hour CO design values to the standards in effect at that time. The current CO design values are defined as the highest of the second-highest concentrations over a two-year period. These data show that maximum design values have declined steadily and have been well below the NAAQS since at least the 1990s.

New Hampshire 1-hour carbon monoxide design  values

New Hampshire 8-hour carbon monoxide design values