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As discussed above , the only historical data on air quality in the County is for CO measured in Chapel Hill . There have been no exceedances above the CO ambient air quality standards at the Chapel Hill site during the last three years . For 1998 , the maximum 1 -hr CO measurement was 8 . 0 ppm, with no observations above the 35 ppm secondary standard . The maximum 8 -hr measurement was 4 . 3 ppm, but there were no observations above the 9 ppm primary standard . There were also no observations above 9 ppm in 1997 or 1996 . The CO data from Chapel Hill are consistent with data for the state . In the 1996 Ambient Air Quality Report, published in April 1998 by the State of North Carolina Division of Air Quality Ambient Monitoring Section, there were no exceedances of the CO ambient air quality standards in the state of North Carolina in 1996 . Data reported in the 1996 Ambient Air Quality Report, showed no exceedances of the PM , o , S02 , NO2 , or lead standards in the state of North Carolina during 1996 . The 1972 - 1995 Ambient Air Quality Trends Summary, published in May 1998 by the North Carolina Department of Environment and Natural Resources , reports that levels of total suspended particles , PM10 , carbon monoxide , sulfur dioxide , and lead in ambient air in the state of North Carolina have dropped substantially since the 1970s . According to the report, the long term trends are less clear for ozone and nitrogen dioxide , both of which have remained fairly constant . There is no reason to believe that the air quality in Orange County did not follow the same trends for these pollutants . Recent ozone data, available from the Division of Air Quality Internet site under the NC Ozone Forecast Center page , suggests that ozone exceedances are increasing . Data can be retrieved for each monitoring location in the state . Recent data for the two monitoring sites adjacent to Orange County and a site in Wake County are summarized in Table 2 - 3 . The table also includes the number of exceedances measured in the Triangle with all nine monitors that the State operates in the Triangle MSA . As shown in the table , the number of exceedances was high in 1998 . Tabulated data were not available for 1999 . Table 2 - 3 . Trends in Ozone Exceedances at Sites in the Triangle Region Site Number of Exceedances 1995 1996 1997 1998 Chatham County 4 3 8 9 Durham - Duke Street 2 4 3 17 Wake County - WRAL Tower 6 5 15 20 Triangle - Based on all nine monitors 16 19 25 44 No trend data are available for PM2 .5 because data are just starting to be collected . Fortunately , there is a monitoring location in Chapel Hill that will generate data for future trend analyses . 5