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The paradox of peak-based ozone air pollution standards

The paradox of peak-based ozone air pollution standards

Houston in 2002. Michael Bludworth/flickr, CC BY-NC

“And when she was good, she was very, very good,

But when she was bad she was horrid.”

Henry Wadsworth Longfellow’s poem may have befit Houston’s air around 1999, when the city briefly ranked as the “New U.S. Smog Capital.” Back then, leaks and flares from local petrochemical plants too often contributed to ozone spikes that could fairly be described as “horrid.” Texas Air Quality Studies in 2000 and 2006 illuminated how industrial and urban emissions interacted to cause these pollution spikes.

Emission control efforts informed by those studies have made Houston smog episodes far less frequent and far less “horrid.” The efforts have been so successful that, averaging in days when our air is “very, very good,” Houston now has lower average ozone levels than most of the country. The city also meets all EPA standards for far deadlier particulate matter (PM) pollution, though benzene and other air toxics remain a concern in some neighborhoods.

Yet Houston ranks sixth smoggiest among U.S. cities in the American Lung Association (ALA) ozone rankings, and likely remains over a decade away from attaining EPA’s recently tightened ozone standards, which were introduced in October 2015.

Why does a city with such clean average air quality remain stuck on lists of smoggiest cities and regions that fail to meet air quality rules? Houston does have some unique air quality conditions. But its situation illustrates a paradox in how we use peak pollution levels to rank cities on air quality and set national ozone standards.


ALA rankings and EPA regulations focus on days with the worst ozone each year. By those measures, Houston is indeed far too polluted. When hot stagnant conditions set in, emissions uniquely concentrated in Houston’s petrochemical industry combine with typical big-city vehicle exhaust to form unhealthful levels of ozone. On such days, monitors downwind of the urban core and the Houston Ship Channel may measure ozone levels exceeding EPA standards.

Air quality from industry, such as oil refineries, combines with exhaust from cars and trucks to affect local air quality conditions. Richard Carson/Reuters

Just four days per year above 70 parts per billion (ppb) at a single monitor is enough to bring an entire region into nonattainment of EPA’s recently tightened standard. Some portion of the Houston region surpasses this limit anywhere from 12 to 36 days each year. Ozone at these levels has been shown to impair lung function and trigger other adverse health outcomes.

From 2013-2015, areas in the U.S. with the fourth-worst ozone level, measured over eight-hour periods, at the region’s most polluted monitor (EPA’s basis for determining attainment) was 80 ppb when averaged over three years. That’s far lower than levels at the turn of the century (Figure 1), but still 10 ppb above EPA’s new limit.

Figure 1. Fourth highest annual May-September 8-hour ozone concentration in 2000-2002 (left) and 2013-2015 (right). Data from EPA, Author provided

Ideally no days would exceed 70 ppb. However, complexities of atmospheric chemistry along with background pollution cause ozone to be relatively insensitive to changes in emissions of its precursors, nitrogen oxides and hydrocarbons. Ozone forms when sunlight drives chemical reactions of these gases.

Greater than 50 percent reductions in nitrogen oxide emissions – which come mostly from vehicle and industrial sources – may be needed to cut peak ozone by a mere 10 percent, and may do little to improve ozone on nonpeak days when biogenic hydrocarbons from trees are less abundant. The response of ozone to emissions changes is less than one-to-one because some ozone originates from background sources upwind, and because the chemistry is nonlinear.

Also, the focus of EPA and ALA on peak day ozone ignores the fact that air quality affects health year-round, not just when the standard is exceeded. Epidemiologists find that health continues to improve as ozone falls far below regulated levels. Regulating just the peaks detracts attention from influences on average ozone conditions.

Air quality duality

Few Houstonians realize our abundance of “good” air days on EPA’s Air Quality Index. The cleanest days occur when gulf breezes bring in air with very little ozone and other pollutants. Even in the summer “ozone season,” we enjoy many days with ozone levels less than a third of the EPA limit.

Though scientists are still grappling with the halogen chemistry and other factors that cause gulf air to be so clean, Houston monitors unequivocally show clean air even on many hot summer days. Those clean-air days help Houston rank lower than most cities and even many rural parts of the country for our average ozone levels (Figure 2).

Figure 2. Average annual May-September 8-hour ozone concentration in 2000-2002 (left) and 2013-2015 (right). Data from EPA, Author provided

The duality of Houston as a “smog capital” and “clean air haven” is illustrated by contrasting the city’s high peak ozone levels with its unusually clean average conditions (Figure 3). This helps explain why Houston fails to attain peak-based ALA targets and EPA standards yet has relatively low rates of ozone-induced premature mortality, which EPA assumes to be linked primarily to average conditions.

Figure 3. Average (blue) and fourth highest (red) 8-hour ozone concentrations in U.S. cities, May-September 2013-2015. Data from EPA, Author provided

This difference between peak versus average has public health implications.

Our analysis of EPA data shows that ozone trends across the country look very different on average and peak-day bases. Ozone has been trending down nationwide far more quickly on peak days than for average conditions, as shown by EPA scientists and confirmed by our figures. Many cities have shown no significant improvement in clean-day or average ozone conditions despite substantial reductions in peaks, though Houston averages have improved.

Here in Houston, while more improvements are still needed, we should breathe easier knowing that our “horrid” smog days are behind us as a result of more stringent emissions controls. With our relatively low average ozone and PM levels, Houston can set aside its outdated moniker as a smog capital.

Nationally, focusing on peak-day ozone too narrowly could constrain us from the broader perspective needed to holistically address our biggest air and climate challenges. Although ozone is harmful, particulate matter (PM) is a deadlier pollutant and climate change poses the greatest environmental challenge to society. Reducing our reliance on fossil fuels overall, rather than exclusively targeting ozone-forming gases, will be needed to jointly address these challenges.