New York City’s smoke-clogged air reached 407 on the Environmental Protection Agency’s 500-point Air Quality Index at one point this week, signifying that pollution levels were “hazardous” and at historically dangerous levels.
But it’s not the worst air quality that the United States has seen. There have been about 40 times over the past decade when the index has risen above 500, into what the E.P.A. calls “Beyond the A.Q.I.” Most of those instances have occurred in Western states, including California, Oregon and Washington, the E.P.A. said in an email, as wildfires spread a blanket of smoke over parts of the region.
The E.P.A. in 1999 released the current version of the six-tier index as a way to communicate to the public the density of five pollutants. A rating anywhere between 301 and 500 is considered “hazardous,” and air quality at that level will trigger health warnings. At that level and beyond, everyone should stay indoors and reduce activity levels. “Use the same information that is for the ‘hazardous’ category,” the E.P.A. advises.
While the A.Q.I. measurement used in the United States does not support values above 500, such values occur so infrequently that the issue rarely comes up, said Robert Rohde, the lead scientist at Berkeley Earth, an organization focused on environmental data science.
“Such levels do occur more often in some foreign countries, such as India,” Dr. Rohde said. Some third-party air quality tracking platforms extend the U.S. A.Q.I. scale and track figures over 500, he added.
Harshal Salve, a professor at the All India Institute Of Medical Sciences in New Delhi, said that as air quality reaches hazardous levels close to or above 500 on the U.S. A.Q.I. scale, people will experience symptoms of respiratory illnesses, such as coughing and a burning sensation in their eyes, with people over 65 years and under five years at the highest risk.
The negative effects can continue even after air quality levels have improved, since pollution particles can cause inflammation of the lung tissue and increase the vulnerability to infections.
By Friday, the cities with the highest A.Q.I.s were seeing improved air quality, with Susquehanna Valley, Pa., at 150, down from 448 on Thursday morning, the highest of anywhere in the United States, according to AirNow, a source for air quality data. Levels below 100 are considered to be below the level known to cause adverse health effects.
The E.P.A. said it has proposed changes to the A.Q.I. to make it more accurately reflect recent scientific studies about particle pollution and health, and to improve the quality of monitoring data. The agency said air quality in the United States is generally improving, even if climate change is contributing to more frequent and severe wildfires.
Olivia Clifton, an atmospheric scientist at NASA Goddard Institute for Space Studies, said that if air quality levels like the ones this week become more frequent, the E.P.A. should consider defining what A.Q.I. levels above 500 say about air quality for the public.
“Is this still hazardous or beyond hazardous?” Dr. Clifton said. “What is the qualitative description of the air quality that would need to be described for a higher A.Q.I.?”
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