Just when I was getting hopeful about the dramatic improvement this year in Beijing’s PM2.5 levels, along comes a summer deluge of local press coverage about what they’re claiming is air pollution’s number two threat: ground-level ozone (臭氧). Recent press reports and a campaign by the China National Environmental Monitoring Center have highlighted the fact that on some hot, sunny days this summer, the worst pollutant has been ozone and not the usual PM2.5. But is ozone really as much a threat to our health as the well-documented PM2.5? And if so, do our usual masks and indoor air purifiers help in any way?

Most of us know ozone when discussing global warming, as the thinning ozone layer in our upper atmosphere protects us here on the ground. But at ground level, ozone is quite destructive to our lungs, causing both immediate and long-term harm as well as increased death rates. Ozone levels are always highest during sunny summer afternoons, as the sun’s ultraviolet light creates ozone from airborne chemicals, especially from vehicle exhaust — most notoriously from old diesel trucks spewing out their toxic cocktail of fumes. The World Health Organization’s Air Quality Guidelines from 2005 recommend safest ozone levels under 100 ug/m3 (using 8-hour exposure limits), while over 160 ug/m3 would be considered unhealthy for vulnerable people such as children and people with lung disease, and over 240 ug/m3 especially unhealthy to all people. China’s MEP guidelines follow the WHO’s interim target of 160 ug/m3, while the US EPA’s AQI “green zone” is under 120 ug/m3. During this week’s heat wave in Beijing, afternoon ozone regularly peaked over all three thresholds, commonly over 200 ug/m3 and peaking at 299 ug/m3. The Los Angeles area, most polluted in the USA, still regularly has ozone days over 200 ug/m3 but is far better than a few decades ago when levels frequently hit over 400 ug/m3. So while ozone levels in Beijing aren’t comparatively as high as PM2.5 is, certainly they are frequently at levels that would be considered unhealthy.

In terms of symptoms, ozone seems to cause much more immediate effects than PM2.5 does. You may have noticed while outside during this hot summer that your eyes sting, your head pounds, your throat burns, you cough and feel a bit short of breath. I’ve certainly had such symptoms this summer during some of my bike rides to my clinic, especially returning home early evening, which is usually when ozone peaks. I’m fortunate that I haven’t had more serious symptoms such as severe asthma flares, but I definitely worry about children’s exposure to ozone, especially those with asthma who live near busy roads in sunny places. Healthy children are also at risk; one sobering study in 2002 followed a group of healthy students around smoggy southern California, and those who spent the most hours outside playing sports had a three-fold increased risk of developing asthma from ozone exposure, compared to those who mostly stayed indoors.

But which pollutant is truly more serious, PM2.5 or ozone? Air pollution action plans for schools (such as the one below) across the USA rank PM2.5 and ozone as equally dangerous pollutants for children, and an AQI over 200 for either PM2.5 or ozone (equivalent to 8-hour ozone of 115 ppb, or 225 ug/m3) would warrant all children staying indoors; for those with asthma, a stricter cutoff AQI of 100-150 could be even more protective:

But does any of this mean we really should change our behavior here in Beijing, any differently than we already act right now with PM2.5? I’m honestly not too convinced; while the WHO Guidelines and a more recent US EPA review do mention data showing increased mortality from higher ozone, and quite strong connections with lung disease, the long-term data just isn’t as impressive to me as it is for PM2.5. For example, PM2.5 clearly causes cancers, and is officially on the WHO’s list of carcinogens, while ozone’s data shows no strong correlation to cancers. Also, PM2.5’s direct damage to the heart is extremely well documented, while data for ozone is far less certain of a connection.

But while ozone’s long-term risks may be less than PM2.5, short-term symptoms seem to affect all of us much more easily, especially right now during summer. Perhaps in this case any one of us who is at higher risk, or who commutes in high ozone areas — that includes anyone sitting on a bus, taxi or subway — could consider an anti-ozone mask, especially if you’ve already noticed symptoms.  Also, anyone who feels symptoms when ozone is high, even if you are young and healthy, should certainly consider using an anti-ozone mask.

But the main problem here is that most masks’ filter material against PM2.5 is quite impotent against the far smaller ozone molecules, reducing ozone by only one-third. The only way to effectively remove ozone via a mask is to add a layer of carbon to the mask. These specialized anti-ozone masks using carbon were initially designed especially for welders, not consumers. Welding is a particularly dangerous occupation as the heat can create dangerously high levels of ozone (over 1,200 parts per billion, more than 12 times the US OSHA standard), and welders have much higher rates of lung disease than the average population, including asthma. Based on this specialized technology, a few consumer-level carbon masks are available. Studies have shown that disposable masks with carbon (in this case, the 3M 9913, also 90% effective against PM2.5), if fitted properly, can eliminate up to 98% of ozone, which I feel is quite impressive. Even more importantly, lung function was stable with the carbon mask but decreased with the no-carbon mask. After 40 hours of use, the carbon filter still worked perfectly, which means a person like myself could wear these masks for at least a couple of weeks of normal commuting activity, and simply throw them away once they get too dirty — or the rubber strap breaks, which usually happens first.

Indoor Air

Ozone inside a house usually is only 40-50% as much as outdoors, but since we all spend much more time indoors than out, it’s estimated that 25-60% of our total exposure to ozone comes from indoors. Fortunately, the answer to this problem is one you’re probably already taking care of: an indoor air purifier. You may have noticed that most of the decent indoor air purifiers already include a carbon filter (活性炭滤网) along with a true HEPA filter (HEPA过滤网) which eliminates PM2.5. Ozone indoors is still probably a less serious hazard to most of us than PM2.5, as well as from dangerous gases like benzene (甲苯) and formaldehyde (甲醛), which causes cancer. These and other volatile gases are frequently elevated in newly renovated Chinese apartments with poorly made furniture, walls and flooring. The carbon filters can work extremely well against ozone as well as all of these gases.

My Bottom Line

I recently had pretty bad asthma attacks, which are now resolved, but I’m still quite nervous about recurrence. So while I’m relieved that I haven’t had any asthma spells this summer, all this recent press about ozone has empowered me to finally get up to speed on ozone research. I’ve decided that the only life change I needed to make was to use a carbon mask (combined with N95) during bad ozone days when commuting to work. Now when I glance at my pollution app on my mobile phone, I will look at ozone concentration as well as PM2.5. Otherwise, I’m already safe at home as my indoor air purifiers already include both carbon and HEPA filters.

But I’m also more cautious this summer with my children and asthmatic patients in clinic, making sure they’re also more aware of ozone’s specific threats to them on these hot sunny days. As usual, living healthy in Beijing always requires a combination of common sense and education on public health issues. And with ozone, just like PM2.5, a bit of the same preventive strategies should keep most of us healthy.

UPDATE 2024: Please follow me at my new website, DrSaintCyr.com. Also my new YouTube channel youtube.com/@drsaintcyr