ACADIA NAT’L PARK — While ground-level ozone readings at the monitors on McFarland’s Hill and Cadillac reached unhealthy levels on both Sunday and Monday, the ozone levels alone only partially caused the unhealthy air. Multiple factors – high moderate levels of ozone, moderate levels of particle pollution, high heat index, and high temperatures and humidity – all combined to make the air in Acadia and on Mount Desert Island unhealthy on Sunday and Monday, said Bill Gawley, Acadia’s air quality specialist.
The Maine Department of Environmental Protection (DEP) recommends that when ground-level ozone readings exceed 70 parts per billion (ppb), the very young, the elderly or those with asthma, respiratory problems or heart disease should reduce prolonged or strenuous activity. But this week, because of the combination of factors, the park and the DEP issued air quality alerts, recommending that most people limit outdoor activity, take a number of breaks and stay hydrated.
To determine unhealthy ozone levels, the Environmental Protection Agency (EPA) averages readings over an eight-hour period. Since October 2015, air is considered unhealthy when the eight-hour average exceeds 70 ppb.
Preliminary data at McFarland’s Hill on Sunday show readings exceeding this threshold for eight hours with an average of 76 ppb. On Monday, there were only four hours – at 1 a.m. and from 5-7 p.m. – over 70 ppb.
On Cadillac on Sunday, ozone levels exceeded 70 ppb for nine hours from 4 p.m. to 12 a.m. with an eight-hour average of 81.5 ppb while Monday’s readings were slightly better, with only five hours over 70 ppb for an eight-hour average of 74.7. On Tuesday, readings at both sites did not exceed 60 ppb, but the heat, humidity and heat index still made people feel uncomfortable.
The current standard of 70 ppb over eight hours, adopted in 2015, is considerably more stringent since EPA began its monitoring program in 1981. In 2008, the standard was lowered from 84 ppb averaged over eight hours to an eight-hour average of 75 ppb. From 2001 to 2008, the standard for unhealthy air was 101 ppb.
That air quality has continued to improve over the last 40 years can be attributed not only to provisions of the 1973 Clean Air Act and the substantive amendments issued in 1990 but also to the EPA’s implementing more stringent standards based on continuous data such as the park’s, said Gawley.
According to historical data compiled by the New England EPA, in 1988, Maine would have had 40 days that exceeded the 75 ppb standard and 49 days that would have exceeded the current standard of 70 ppb. Historically, 1988 is the year with the highest number of exceedances in Maine, but the table listing the number of historical exceedances does not break out the numbers by region, and National Park Service data does not include data for 1988.
In Acadia, ground-level ozone forms in the presence of high temperatures, wind from the southwest and a high-pressure system that stalls the polluted air mass. The majority of ground-level ozone coming into Maine arrives on plumes of air from areas south and west of here.
According to an EPA fact sheet, the 2015 standards were based on extensive scientific evidence about ozone’s effect on public health. In addition to protecting public health, the new standards were also expected to protect trees, plants and ecosystems.
That fact sheet also indicated that reducing air pollution for children and adults to 70 ppb would result in public health benefits worth an estimated $2.9-5.9 billion. These benefits outweighed the cost of $1.4 billion which was the estimate for taking measures to comply with the new standard. The Clean Air Act requires EPA to review the standards every five years.