Do ‘ozone action days’ actually inspire people to act?

robert wyly cleveland pollution

Industrial pollution obscures Cleveland’s cityscape in this 1960 photo from Robert Wyly (courtesy of Elvin Wyly).

“Ozone: Good up high, Bad nearby.” So goes the U.S. EPA’s catchy (?) refrain to help people distinguish between (good) atmospheric and (bad) ground-level ozone.

Fortunately, we have gotten some good news on the former in the past few days. A team of researchers has concluded that we are finally building up more good ozone; that is, the massive hole in the protective ozone layer over Antarctica is finally beginning to heal thanks to the phasing out of chlorofluorocarbons under the 1987 Montreal Protocol. It seems like the ozone layer may be on course to fully recover by the middle of the century.

Unfortunately, the news is not as great on the latter front, as we are also seeing an increase in ground-level ozone. On Tuesday, NOACA issued an ozone advisory, warning residents of Northeast Ohio that ambient levels of ground-level ozone may reach harmful levels, which U.S. EPA defines as those above 70 parts per billion (ppb).

As I’ve documented before, the number of these advisories has dropped significantly over the past decade; however, this summer’s hot, dry weather has stymied that downward trend somewhat. Tuesday marked the seventh time this year that ozone levels in the region exceeded 70 ppb, and NOACA encouraged people – particularly sensitive populations like the elderly and those with respiratory conditions – to limit their time outdoors during afternoon and evening hours in order to minimize their exposure. These types of warnings are commonplace; officials in some 230 metropolitan areas release similar advisories.

Air quality alerts as a call to action?

In a number of areas, these advisories are dubbed action days, highlighting the fact that the agencies see them as a call to arms around ozone pollution. Air quality officials are pushing citizens to not only limit their personal exposure to pollution but also to take steps to reduce the amount of ozone precursor emissions within the region. NOACA, for instance, encouraged people to carpool and take public transportation on Tuesday.

It was with all of this in mind that I read a recent post from the Tri-State Transportation Campaign (TSTC), a non-profit group in the New York City metro area that promotes sustainable transportation and alternatives to car dependency.

In a post titled “Air Quality Alert Days Should Be a Call for Better Streets, Not to Stay Indoors,” Emma Kilkelly, TSTC’s Communications Assistant, chided the Connecticut Department of Energy and Environmental Protection (DEEP) for telling residents to limit their time outdoors, rather than changing their transportation modes away from cars. Kilkelly wrote that “this type of messaging addresses only the symptoms, not the cause of air pollution.”

It’s worth examining this argument further. Do ozone alerts focus on minimizing exposure at the expense of promoting sustainable transportation? Is it actually harmful to tell people to avoid high levels of air pollution? And, if you assume these two points, would using ozone alerts to promote active transportation and public transit actually work?

Now if all of this seems like unnecessary semantics, that’s because it is. But I’m a huge nerd who did all of this research, and this is my website God damnit, so I’m going to write about it.

But air quality agencies are already doing just that

First, it’s pretty disingenuous to claim that air quality agencies are not already encouraging people to change their transportation patterns on ozone action days. The very agency that Kilkelly critiques, DEEP, does this. To be fair, she acknowledges this in her post, so the argument rings a bit hollow, prima facie.

Second, there does appear to be value in encouraging people, especially those most vulnerable to the deleterious impacts of high ozone levels, to limit their exposure. We know that short-term spikes in ground-level ozone levels can have significant impacts on public health.

In a landmark 2004 study, Michelle L. Bell and colleagues from Yale University studied the changes in mortality rates tied to daily fluctuations in ozone levels across 95 U.S. cities. They found that for every 10 ppb increase in daily ozone levels, all-cause mortality rates jump by 0.52% during the following week. That number goes up to 0.67% for a 20 ppb increase, which is quite common on ozone exceedance days. And this effect is even more pronounced for elderly Americans. Mortality rates for Americans aged 65 to 74 go up 0.7% for each 10 ppb spike in ozone.

Do air quality alerts benefit public health?

Accordingly, limiting the amount of time that people from higher-risk groups spend outside during the late afternoon and early evening hours can benefit their well-being. But does encouraging people to shift the time that they exercise or go outdoors – so-called avoidance behaviors – actually work?

This answer appears to be yes. Economists Alison L. Sexton Ward and Timothy K. M. Beatty published a paper (paywalled) last year studying this very question. They found that, on air quality alert days, individuals do engage in avoidance behaviors.

Overall, people limit the amount of time they spend outdoors engaged in vigorous physical activity by 18%. The number is even higher among the elderly, who reduced the amount of time by 59%. A similar study (PDF) from Australia noted that cyclists reduce the amount of time they spend biking on air quality alert days by 18.2% for commuting and 38.2% for recreation, respectively.

But can they drive mode shift?

Third, the question of efficacy remains. If air quality agencies devoted all their public outreach efforts to promoting mode shift on ozone action days, would they suceed?

Unfortunately, the evidence suggests they wouldn’t. Several agencies have tried this approach for years. San Francisco, for instance, operates the Spare the Air Program, which encourages alternative commute modes and subsidizes transit passes on exceedance days. The problem is that it doesn’t work.

Multiple studies cast doubt on the effectiveness of Spare the Air. A 2009 study from W. Bowman Carter and Matthew Neidell noted (PDF) a small increase in transit ridership on ozone action days, but it was statistically insignificant. Stephen Sexton, in turn, argues (paywall) that ozone alerts may actually push some transit users to drive to work in order to limit their exposure to higher pollution levels. His review of the program found an increase in both transit trips and VMT.

As a result, Spare the Air apparently “has the perverse effect of increasing car trips,” making it, essentially “a pay-for-pollution program.”

Studies from other cities back up these results. Calvin Tribby and colleagues, for example, looked at the impact of air quality alerts on traffic volumes in Salt Lake City from 2001 to 2011. They concluded (paywall) that “messages regarding air quality and voluntary reductions in vehicle trips are not only ineffective at reducing traffic but apparently increased average daily traffic levels.” Their work again suggests that normal transit users shift to driving in an attempt to limit their personal exposure to pollution.

All told, the weight of the available evidence contradicts Kilkelly’s central argument. While ozone action days may drive people to take certain actions, they are not necessarily the actions she (and I) would like to see. Instead of criticizing air quality officials for encouraging people to take avoidance behaviors on exceedance days, we should acknowledge the limits of what their pleas can realistically achieve.

While ozone alerts are highly unlikely to cut down on VMT, they can and do provide a clear public health benefit. Let’s focus our attention and advocacy on those actors who can actually influence the nature of our transportation system in the long-run. Criticize the underlying system that forces your friendly area air quality planner to issue these advisories in the first place, not her/him for doing so.

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  • Dude, Paris banned cars basically on high air pollution days. That is sort of common internationally and it works perfectly. The problem is that very few people see these air quality alerts because this issue isn’t even registering in the public consciousness. Encouraging people to take transit is a lame half measure that tells people that this isn’t a big deal. That’s not NOACA’s fault but

    • Tim

      Yes, Paris banned cars on high pollution days. And a number of other cities have done similar things, most popularly Mexico City’s hoy no circula program. The problem is that, by and large, they do almost nothing; worse yet, they often make the problem worse. There’s dozens of studies that demonstrate this.

    • Tim

      Resources for the Future has done a ton of research on the issues associated with hoy no circula and other types of vehicle bans. Here’s one post on their underlying issues: