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Massachusetts endured a severe ozone season (May through September) in 2002, when ground-level ozone exceeded the national eight-hour health standard at one or more state monitors on 29 days. This is the greatest number since the standard went into effect late in 1997, eclipsing the previous high of 27 days set in 2001. To find a worse year, one must go back to 1988 when, had the current 8-hour standard existed, 43 such days would have occurred.
Ozone, harmful to breathe in high concentrations, forms in abundance when hydrocarbons, the main ingredients in gasoline and solvents, and nitrogen oxides, which are produced when fuels are burned, chemically react on hot, sunny days.
And hot, sunny days abounded during the 2002 summer, when weather systems aligned to produce numerous stretches of heat and southwesterly winds, which channeled pollutants along the Northeast urban corridor into Massachusetts. During one particularly unrelenting hot spell, from August 10-19, Massachusetts monitors recorded unhealthy ozone levels on nine of the 10 days.
The chart below depicts the number of days in recent years when Massachusetts? monitors exceeded the 8-hour and the older, less stringent 1-hour national ozone standards. Because these year-to-year changes are largely due to meteorological variability, long-term trends are difficult to gauge. While the 2002 summer featured many unhealthy air days, it was only two years ago, in 2000, that Massachusetts experienced its cleanest ozone year on record.

Elevated ozone levels were, of course, not confined to Massachusetts but were spread over wide areas of the eastern U.S. See EPA AIRNow for daily local and regional ozone maps.
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