Ozone levels have fallen dramatically, though you probably didn’t notice

cleveland skyline smog

Smog obscures the Cleveland skyline in this picture from July 20, 1973 (courtesy of the National Archives/U.S. EPA).


As someone who has spent most of his life in the city of Cleveland and bikes to work across the Lorain-Carnegie Bridge on a daily basis, I feel like I have a close, personal relationship with air pollution here.

I can tell when the steel mills and other factories in the Industrial Flats are releasing more sulfur dioxide (SO2) than normal from the distinctive odor of rotten eggs. I have entirely too much experience trying to avoid the clouds of diesel particulate matter as they belch forth from GCRTA’s older buses. I have inhaled more than my fair share of nitrogen dioxide (NO2) from passing vehicles.

The dynamics of ground-level ozone

But one common urban pollutant that I cannot and will never be able to smell or see or taste is ground-level ozone. It is completely colorless and odorless. The only way you can notice ozone is from afar, as it helps obscure your view of cities on particularly hazy days. But even then, you can’t really “see” it, as the ozone is just one component of the smog that envelops cities.

Ozone is a sneaky little bastard. It forms above us in the troposphere, travels dozens to hundreds of miles downwind, and then silently works its way into our airways. Only when you have already inhaled it can you possibly begin to notice ozone, as it irritates and inflames tissue in your nose and lungs.

Fortunately, thanks largely to regulations put in place over the past several years by the U.S. EPA, ozone levels have been falling consistently around the country. According to EPA, ozone declined by one-third nationwide, from 1980 to 2014.

But while long-term ozone concentrations certainly affect public health, environmental and public health officials typically focus more on the impacts of spikes in the pollutant over the shorter term. The short-term health effects of rising ozone levels can be significant. According to a landmark 2004 study from Michelle Bell and colleagues, when ozone increases by 10 parts per billion (ppb), mortality rates in Cleveland increase by roughly 1% during the next week. These daily spikes also lead to additional hospitalizations, missed school days, and missed workdays due to asthma and other respiratory conditions.

For these reasons, U.S. EPA requires local officials to monitor ozone and advise the public when they project that ambient levels are expected to exceed 70 ppb. Unfortunately, the Cleveland area has already experienced three days this year on which concentrations exceed 70 ppb. Two of these occurred last week, given that air temperatures increased significantly as high pressure moved into the region.

Yet, as NASA pointed out recently, reductions in emissions of ozone precursors – namely nitric oxides (NOx) and volatile organic compounds (VOCs) – have gone a long way towards limiting the number of exceedance days over the past few years. Without these emissions reductions, Cleveland would have experienced roughly 4-5 more exceedance days in 2011 than we actually did, as the map below shows.

ozone exceedance days avoided 2011

Ozone exceedance days avoided in 2011 as a result of emissions reductions over the past decade (courtesy of NASA).

The benefits of this reduction are tangible, in both blood and treasure. But, at a more basic level, it provides greater peace of mind for all of us. Parents no longer have to worry as much about keeping their children indoors to protect them from pollution. Those of us with asthma don’t have to think about altering our behavior to spend less time outside.

Despite the hype, ozone levels are declining

Given the recent media coverage about worsening air quality worldwide, the fact that ozone levels continue to decline throughout most of the U.S. may come as something of a surprise. I mean, the American Lung Association just gave Cleveland an F for air quality a month ago. But, when you actually get beyond the sensationalized headlines and dig into the data, you’ll find that our air is cleaner than it is has ever been, and it is far cleaner than it was even a decade ago.

Now, none of this should be taken to mean that we can get complacent or that air quality is no longer a pressing challenge; nothing could be further from the truth. I would venture that there are relatively few people more concerned about or aware of air quality issues in this region than I, but I am also among the first to acknowledge the progress we have made and continue to make. But don’t take my word for it. Let’s actually look at the data.

Perhaps the easiest way to chart changes in ozone, over time, would be to look at the average daily ozone levels for the region. In order to do so, I collected data on daily ambient ozone concentrations for Northeast Ohio from 2005-2015 from EPA’s Air Quality System (AQS). This is charted below.

mean annual o3 level 2005-2015

Mean daily ozone levels in Northeast Ohio from 2005-2015.

While there appears to be a fairly small – but steady – decline since 2005, this is not necessarily the most valuable metric to use. First, because ozone  is a secondary pollutant, it is highly dependent upon weather conditions to form. This means that ozone levels can vary dramatically from one day to another, based upon ambient temperatures or whether or not it is raining. Secondly, there is relatively little reliable science on the health impacts of ozone at levels below 50 ppb.

The number of ozone exceedance days has fallen considerably

A more accurate way to account for changes in ozone levels is to examine the number of exceedance days per year. But, because EPA continues to update the National Ambient Air Quality Standard (NAAQS) to reflect changes in science, this does not give us a true apples-to-apples comparison. It wouldn’t be accurate, for example, to claim that air quality did not improve from 2000 to 2015 if a city had 10 exceedance days in each year, given that the NAAQS was 85 ppb during the former year and 75 ppb during the latter.

Because there are several ozone monitors operating in the region, I took the highest daily ozone value from among these monitors and used that as the regional value for a given day. To get a true comparison, I counted day as an exceedance if at least one monitor within the 8-county region registered a value of 71 ppb or more, given that the current NAAQS is 70 ppb.

annual o3 exceedance days 2005-2015

Number of ozone exceedance days per year in Northeast Ohio from 2005-2015, using a 70 ppb cutoff.

As you can see, there has been a nearly precipitous decline in the number of exceedance days over the past decade. While there is some interannual variation, based upon weather (e.g. 2012), the overall trend is undeniable. While the region averaged 43.3 exceedance days per year from 2005-2007, that number fell to just 7 per year from 2013-2015.

Another way to frame changes in ozone levels is to consider the average ozone concentration within the region on a given exceedance day. It may be more harmful for public health to have 10 exceedances with an average concentration of 80 ppb than to have 15 exceedances that average 71 ppb. Fortunately, this metric has also declined significantly since 2005. While the data are fairly noisy, they also demonstrate a strong overlap with the number of exceedance days per year. In other words, during years when we have more exceedances, ozone levels on those days tend to be higher.

mean annual o3 exceedance level 2005-2015

Mean annual ozone exceedance level per year from 2000-2015.

Clearly, by basically any measure, ozone levels have fallen considerably in the region over the past several years, which has directly enhanced public health and well being. In a 2013 study, EPA scientists Neal Fann and David Risely estimated the nationwide public health benefits due to decreases in ozone concentrations from 2000 to 2007. During this period, a 3.5 ppb decrease in national ozone levels prevented between 880 and 4,100 premature deaths. Northeast Ohio, in particular, benefited from this trend; Cuyahoga County avoided more than 30 premature deaths per year during this period, more than all but a handful of counties in the country.

But climate change threatens this trend

But, as I’ve noted before, climate change threatens to stymie this progress. Rising temperatures and changes in precipitation and wind patterns may create conditions more favorable to ozone formation in the future.  Based on a recent EPA report, ozone levels may spike by 1 to 5 ppb, depending on much surface temperatures increase. To account for this effect, I identified those days from 2005 to 2015 on which ozone concentrations peaked between 66 and 70 ppb. As the chart below illustrates, the number of exceedance days would have increased markedly during this period, if the temperature increases associated with climate change had already taken effect. On average, there would have been an additional 13.9 exceedance days per year, ranging from a low of 4 in 2009 to a high of 29 in 2006.

o3 exceedance days with & without climate change

The number of ozone exceedance days in Northeast Ohio from 2005-2015 before and after accounting for the impacts of climate change.

The system works, if you let it

Ultimately, these trends point to a clear conclusion – the air pollution control system in this country works. Donald Trump may want to ban the EPA, but – and this is shocking, I know – I’m going to go ahead and call bullshit on his claim that “we’ll be fine with the environment” afterwards. The clear improvement in air quality that we have seen in this country would not have been possible without the passage of the 1970 Clean Air Act Amendments or the creation of the EPA, which has enforced them. We are all the beneficiaries of the system that has been in place over the past four-plus decades.

But this progress is not a given. As we’ve seen, climate change – itself a product of air pollution – threatens to harm air quality in the long-term. If we get complacent or, worse yet, try to roll back these gains, we will all suffer. Ozone is a fickle and complicated bastard that can strike where and when you are not expecting it. Let’s not give it that chance.

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