Why strict new Texas ozone rules won’t do much for citizens’ health

Texas ozone rules imposed by EPA

By Daniel Cohan

Let’s suppose EPA is right that cities like Houston and Dallas-Fort Worth can lower their ozone levels to 70 parts per billion (ppb) by 2025. (According to my calculations, that’s impossible. But let’s suppose.) How much would people’s health benefit?

The answer, sadly, is: Not much.

To understand why, you have to understand how states report air pollution to EPA. Actual reductions in the pollution that we breathe day-to-day tend to be far less than reductions in the “design values” that determine attainment.

When we hear that Houston’s current “design value” is 80 ppb, that doesn’t mean we’re breathing 80 ppb ozone all the time. Instead, it means that we’re breathing 80 ppb on the fourth-highest eight-hour ozone observation period of the year, averaged over three years, at the most polluted monitor in the eight-county Houston-Galveston-Brazoria region.

If just one monitor registers more than the new 70 ppb standard, the whole region remains in nonattainment. That’s one reason why I believe Houston and Dallas can’t attain 70 ppb by 2025.

But my focus today is on the health benefits of reaching the stringent new standard. Let’s say it’s possible. Would it even do that much good?

 

When states develop plans for attaining the ozone standard, they focus on controlling the most polluted monitors on days with the most smog. As the adage goes, “What gets regulated gets done.” Since the fourth-worst ozone day at the most polluted monitors determine attainment, it makes sense to target efforts toward reductions on those days and locations.

However, the emissions controls that clean up ozone on the dirtiest days do far less to reduce ozone at other times.

Ozone is not emitted directly. Instead, it forms from reactions involving sunlight acting on two main ingredients — nitrogen oxides (NOx) and hydrocarbons. Ozone requires both ingredients, just as bread requires both flour and yeast. To cut ozone formation, we need to cut the limiting ingredient – just as removing yeast could keep any bread from rising, regardless of the flour supply.

Trees give off large amounts of hydrocarbons, especially when it’s sunny and hot. Thus, the hot, sunny days that favor ozone formation also cause trees to emit higher rates of biogenic hydrocarbons. That means on warm, high-ozone days, the limiting ingredient — the ingredient to cut — will be NOx.

Because ozone regulations focus on the peak days, NOx controls dominate attainment efforts. Due to quirks of ozone chemistry, very deep cuts in NOx are often needed to achieve modest cuts in ozone. In fact, EPA and others forecast that some regions may need to cut NOx by half or more to reduce peak ozone by 10 ppb. As I wrote recently, EPA estimates drastic controls of 121,000 tons NOx and 20,000 tons VOC are needed to lower Houston’s ozone design value just 5 ppb in 2025 to meet the requisite 70 ppb.

But away from the peaks — aside from the year’s hottest days — ozone tends to be far less sensitive to NOx, and even varies inversely with NOx under certain conditions. Thus, if peak-day ozone drops 10 ppb under a NOx-dominated strategy, ozone levels on typical days will drop far less, and ozone on clean-air days may actually rise. Heather Simon and her colleagues at EPA have found exactly that sort of trend at most ozone monitors in the U.S. since peak-focused ozone controls began.

That especially matters in Houston. Overall, Houston has remarkably low ozone – lower even than rural Texas, New Mexico and Utah, since our bad ozone days are balanced by lots of clean days with Gulf air.

 

If ozone levels below 70 ppb caused no health problems, it would make sense to focus single-mindedly on the peaks. But epidemiologists tell us that’s not likely the case: Numerous epidemiological studies show daily ozone and death rates correlate almost linearly down to concentrations far below 70 ppb.

But away from the peaks — aside from the year’s hottest days — ozone tends to be far less sensitive to NOx, and even varies inversely with NOx under certain conditions. Thus, if peak-day ozone drops 10 ppb under a NOx-dominated strategy, ozone levels on typical days will drop far less, and ozone on clean-air days may actually rise. Heather Simon and her colleagues at EPA have found exactly that sort of trend at most ozone monitors in the U.S. since peak-focused ozone controls began.

That especially matters in Houston. Overall, Houston has remarkably low ozone – lower even than rural Texas, New Mexico and Utah, since our bad ozone days are balanced by lots of clean days with Gulf air.

If ozone levels below 70 ppb caused no health problems, it would make sense to focus single-mindedly on the peaks. But epidemiologists tell us that’s not likely the case: Numerous epidemiological studies show daily ozone and death rates correlate almost linearly down to concentrations far below 70 ppb.

In other words, it’s possible that the health benefits of lowering peak-day ozone from 80 to 70 ppb could be offset by a rise from 50 to 60 ppb on a clean-air day.

And what about regular, nothing-special days? Due to the nonlinearities of ozone chemistry, EPA estimates (see Figure 3-6) that the 10 ppb reduction in ozone needed for Houston to meet a 70 ppb standard would curtail mean ozone in Houston by just 2 ppb. Health benefits would be correspondingly small.

Put together, the utter lack of leverage in achieving ozone health benefits from NOx controls is stunning. We need NOx cuts of more than 50 percent to get just cuts of just 10 ppb ozone peaks. And those 10 ppb cuts in the peaks yield cuts of only a few ppb in the mean. The air on regular days — not the best, not the worst — wouldn’t change much.

That explains why health measures didn’t respond much to the tougher ozone standard. In a little noticed figure on page ES-9 of EPA’s Health Risk and Exposure Assessment for Ozone, EPA projects that cutting the peak-based ozone standard from 75 to 70 ppb (6.7 percent) would cut ozone-related mortality by less than 2 percent. Again, that’s because cleaner days don’t respond as favorably to cuts in NOx.

Thus, casting aside whether we even can reach 70 ppb, we must also ask if the corresponding cuts in smog impacts are worth it.

Interestingly, despite the profound inverse leverage, EPA’s Regulatory Impact Analysis found the stringent new standard’s health benefits to outweigh costs. But the improvement in health comes more by accident than design: EPA estimates 50 to 75 percent of health benefits will come because measures to cut NOx coincidentally reduce particulate matter, another component of air pollution that’s far more dangerous to human health. But because the pollution standard being set was for ozone, EPA’s Regulatory Impact Analysis provides only a cursory approach to quantifying the benefits of cutting particulate matter.

Is the difficulty of cutting NOx emissions by more than 50 percent worth it to cut ozone’s health impacts by just 2 percent? And if the aim is to cut the more deadly particulate-matter pollution, why are we doing so through an ozone standard, with inadequate analysis of the interaction between ozone and particulate matter? If reducing particulate matter is the aim, there are almost certainly more efficient means to do so.

My broader concern is that the Clean Air Act has entrenched an air-quality standard-setting process in which even slight health impacts reign supreme over tremendous concerns about feasibility and cost. Much as I would like no one to be exposed to too much ozone, particulate matter is likely ten times deadlier. And the greatest environmental risk of all now is greenhouse warming, which is not at all averted by NOx controls. As the Volkswagen diesel scandal reminds us, bypassing certain NOx controls can reduce fuel use and CO2 emissions.

Unlike some, I am not arguing that ozone is anything other than a harmful, deadly pollutant. As a Houstonian, I’m thankful that ozone smog has been brought down so far from peak levels. But life is full of risks and hazards. We live in a city forever threatened by hurricanes and floods, and where people die all too frequently in auto accidents. Sensible risk reduction inevitably involves setting priorities.

With climate change generating a fierce urgency to curb CO2, should boutique cuts in peak ozone really be our priority? Sure, some NOx-cutting measures (e.g., cuts in fossil fuel and fertilizer use) cut global warming too, but others (e.g., Volkswagen diesel NOx controls; selective catalytic reduction; gasoline vs diesel) involve trade-offs between warming and ozone.

It’s not a choice that Houston or Texas can make alone. Under the Clean Air Act, setting standards for ozone is a federal decision. But given reasonable doubts that 70 ppb ozone can be attained, and given the meager benefits of attainment, perhaps enlightened minds can look toward Paris and realize that the toughest environmental challenge of our day is climate, not ozone.

Bending the carbon curve is doable, but it will take all of the can-do spirit of Houston and beyond. Let’s take a deep breath, perhaps with an added touch of ozone, and get to work on our planet’s greatest sustainability need.

Daniel Cohan is an associate professor in the Department of Civil and Environmental Engineering at Rice University. His research specializes in the development of photochemical models and their application to air quality management; uncertainty analysis; energy policy; and health impact studies.

This article first appeared in the Houston Chronicle on December 7, 2015.