De-mystifying the Air Quality Index
It’s a hot summer’s night. Sleep is impossible. To make matters worse, mosquitoes have found a hole in the screen.
If only, say, two mosquitoes get through the screen, the situation might not be too bad. Our “mosquito radar” can track two mosquitoes. Should one light, you’d notice and slap her before she could drink.
Should four mosquitoes get through the hole, human reflexes could probably still handle the situation.
Six mosquitoes, though, might be pushing things, and eight would definitely be a challenge. Distracted by a mosquito exploring your arm, you fail to notice the one dining on your leg.
Many toxins seem to act in a similar way: The body’s defenses – detox and repair systems – can deal with small quantities of poison, but larger quantities overwhelm us.
The ozone AQI – or air quality index – assumes ozone works this way. A certain amount of ozone in the air is deemed not to be harmful; beyond that amount, it becomes dangerous.
Ozone absorbs ultraviolet (UV) radiation.
That is why ozone in the stratosphere, more than 20 miles over our heads, is “good ozone:” It absorbs solar radiation that would otherwise damage DNA and cause cancer.
To measure ozone, instruments shine UV through the air, and measure how much is absorbed. The more absorption, the more ozone there is.
Under federal air regulations, if there are no more than 59 molecules of ozone in a billion molecules of air, air quality is considered “good:” It should have no impact on health.
Regulators feel we can deal with 59 “mosquitoes” in that much air.
Sixty molecules of ozone in a billion molecules of air – 60 parts per billion, 60 ppb – is a “breakpoint,” where air quality is no longer described as good, but as “moderate.” When the air has this much ozone in it, says the EPA, “unusually sensitive people should consider limiting prolonged outdoor exertion.” But for most people, the agency feels levels up to 75 ppb will not have adverse health effects.
Seventy-six ppb is the next breakpoint. Air with 76 molecules of ozone per billion molecules of air (76 ppb) is deemed unhealthy, not for everyone, but for people who are especially sensitive: Asthmatics; people with lung disease; and the elderly.
It’s also unhealthy for anyone exercising. That’s because breathing deeply, we pull more air – and more ozone – into our lungs.
Reporting air quality in parts-per-billion can be cumbersome, so air agencies convert these numbers to “rounder” ones. If there are 59 molecules of ozone for each billion molecules of air, the AQI is reported as 50 – the very upper edge of what’s considered “good.”
If there are 75 ppb, the AQI is 100 – the very upper edge of “moderate.”
A reading between 60 and 75 translates to an AQI between 50 and 100.
And so on.
Those who consider air pollution regulations onerous point out that these are awfully tiny amounts of ozone Ð that 76 ppb (which translates to an AQI of 101, the breakpoint where the air is deemed harmful to sensitive groups) is equivalent to making a martini with an ounce of vermouth in something over a hundred-thousand gallons of gin.
True, but irrelevant. If anything, it demonstrates ozone’s potency as a toxin.
The AQI embodies a couple of assumptions: First, that there is a natural baseline – a level of ozone that would exist in the air even if humanity were driving no cars or trucks, generating no energy, fertilizing no farms, clearing no forests. Second, that this “natural background” level causes no adverse health effects.
Over the years, the AQI calculation has been repeatedly revised to make it more conservative, reflecting accumulating evidence that natural background levels are lower than had been presumed, and ozone more harmful. The most recent revision was made this past March; some say it’s already due for another.