【臭氧的新威脅猶勝PM2.5
Move Over, PM2.5; Ozone is the New Black】
(myhealthbeijing.com, Jul. 21, 2015) —by Richard Saint Cyr MD
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. 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?
就在我才對北京今年上半年的細懸浮微粒污染物(PM2.5)的污染指數有顯著改善此剛剛懷有希望的同時，就讀到一波接連其後的媒體報導提及的空氣中主要威脅的第二順位——也就是地表臭氧。（備註：地表臭氧的污染主要是來自於汽車廢氣裡的氧化物、工廠廢氣以及含有化學物質的煙霧與陽光進行的化學反應。）最近的媒體報導和中國國家環境監測中心所做的調查都明確地指出，這個夏天裡一些炎熱且陽光照射強的日子裡，指數最高的污染物往往是臭氧，而不是平常的PM2.5。不過，臭氧真的會比已被充分研究過的PM2.5更會對人體健康產生危害嗎？如果是的話，我們平時戴的口罩和室內的空氣濾清器會有任何防護的作用嗎？
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.
我們當中大部分人都通過關於全球暖化的討論而對臭氧有所了解。正是由於位於高大氣層中日漸稀薄的臭氧層，我們才免於太陽紫外線輻射的危害。但是在地表附近，臭氧對我們的肺十分具有破壞性，它不僅會導致短期與長期的危害，還會提高死亡率。臭氧的濃度總是在夏天陽光充足的下午達到最高值，因為此時太陽釋放出的紫外線與空氣中的化學物質—特別是那些臭名遠播的舊式柴油卡車所排放的毒氣—產生的反應最活躍。世界衛生組織在2005年公布的空氣品質參考指數中，所建議的安全臭氧濃度應為每立方公尺低於100微克（以暴露在污染環境下8小時為極限）。若臭氧濃度高於每立方公尺160微克，則會對身體健康較為虛弱的小孩和肺病患者產生不良影響。若臭氧濃度高於每立方公尺240微克，則會對所有人都產生危害。中國環境保護局的參考指數是依循了世界衛生組織每立方公尺160微克的暫定目標，而美國環境保護署的空氣品質指數將安全的「綠色區域」定為在每立方公尺120微克以下。北京7月中旬的熱浪中，下午的臭氧濃度經常突破以上所述的三個門檻—一般都在每立方公尺200微克以上，最高達到每立方公尺299微克。在美國的洛杉磯—長期以來作為美國臭氧污染最嚴重的城市—經常有臭氧濃度超過每立方公尺200微克的日子，但已經比幾十年前經常超過每立方公尺400微克的臭氧濃度好得多。所以說，北京的臭氧濃度雖然相比之下沒有PM2.5濃度危險（2014年北京PM2.5濃度的平均值是每立方公尺86微克，比洛杉磯的每立方公尺18微克高很多），但是經常處於不健康的範圍內。
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.
就可能引發的症狀來說，臭氧似乎比PM2.5更快會造成不適的身體反應。當你在炎炎夏日裡外出時，可能會發現眼睛刺痛、頭會抽痛、喉嚨燥熱，你會咳嗽並感覺有點喘不過氣。就我個人而言，我在騎車去診所的路上有時確實為這些症狀所困擾，尤其是在傍晚的下班臭氧分布濃度高峰期間。幸運的是，我還沒有出現像是氣喘發作那樣更嚴重的症狀，不過我的確十分擔心臭氧對兒童的影響，特別是那些住在陽光充足而車水馬龍的大街旁且患有氣喘的孩子們。不僅如此，健康的兒童們也籠罩於危險之中。有一項2002年所做的研究著實發人深省，因為其調查了一群居住在霾霧籠罩的南加州的健康兒童。因暴露在臭氧污染的環境下，那些戶外體育運動時間長的兒童得到氣喘的機率是大部分時間留在室內的兒童的三倍之多。
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.
但是哪一種污染物更嚴重呢，是PM2.5還是臭氧？美國為學校制定的空氣污染行動方案將PM2.5和臭氧列為對兒童同等危險的污染物。無論PM2.5還是臭氧，只要空氣品質指數超過200（相當於8小時暴露下，臭氧濃度達每立方公尺225微克），學校就需要讓所有孩子待在室內。對於有氣喘的兒童來說，該行動方案則建議，更嚴格的空氣污染指數的限制範圍要限縮在100至150之間。
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.
但是這些是否說明我們在北京真的應該改變一下生活習慣了？我們是否應該採取與現在因應PM2.5不同的對策來對付臭氧？說實話，我並不認為我們需要針對臭氧做出相應的改變。儘管世界衛生組織的空氣品質參考指數與美國環境保護署於近期發佈的一項調查顯示，臭氧污染與致死率以及肺病發生率均成正相關，可是在我看來，關於臭氧的長期數據並沒有像PM2.5那樣的令人憂心。例如，PM2.5很明顯地會導致癌症，並正式出現在世界衛生組織的致癌物名單上，然而關於臭氧的數據並沒顯示出其與癌症有很強的關聯性。另外，PM2.5對心臟的直接破壞已經被非常全面地記錄下來，而關於臭氧的數據卻尚未有如此確定的關聯。
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.
不過，雖然從長期看來臭氧並沒有PM2.5那麼危險，它所誘發的短期症狀卻更容易影響我們每一個人，特別是現在正值夏季。具有高風險呼吸道疾病的人們或是在臭氧濃度較高環境下通勤的人們—包含了乘坐公車、計程車還是地鐵的人—都應該要考慮佩戴防臭氧的口罩，尤其是那些已經有發現呼吸道症狀的人。一旦你感覺到高濃度地表臭氧所導致的症狀時，就算你既年輕又健康，也應該考慮佩戴防臭氧的口罩。
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.
但是問題在於，大多數口罩中針對PM2.5的不含碳過濾材質對更為微小的臭氧分子能發揮的作用相當有限，因為這類過濾材質只能除去三分之一的臭氧。用口罩去除臭氧的唯一有效辦法就是在其中加一層碳。此類特製的防臭氧口罩最初是為焊接工人特別設計的，而不是針對普通消費者。焊接是一個格外危險的職業，因為焊接時的高溫可以產生極高濃度的臭氧，其濃度超出美國職業安全與健康署所訂定標準的12倍多。因此，焊接工人的肺病（包括氣喘）發生率要遠高於一般人。市面上出現了幾款利用這種特殊技術的含碳口罩。以上的研究和另外一個更新的研究表明，含碳的可拋棄式口罩—如對PM2.5有90%功效的3M 9913型—可以在正確使用的情況下去除98%的臭氧。我認為這還是很能發揮保護效果的。更重要的是，研究還顯示，含碳口罩使肺功能穩定，不含碳口罩則使肺功能下降。此外，含碳口罩的碳層經過40小時的使用之後，依然可以很有效地過濾雜質。這意味着像我一樣的上班族可以在上下班時戴上這類口罩，每個口罩至少可以使用兩星期。在用髒了之後，或者在大多數情況下都是彈性綁帶先斷了之後，就可以扔掉。
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 along with a true HEPA filter 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 and formaldehyde, 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.
室內的臭氧污染一般只有室外污染程度的40%到50%，可是由於我們在室內的時間比在室外的時間要長很多，一個研究的估算顯示，我們接觸到的25%至60%的臭氧都是來自於室內。幸運的是，解決這個問題的辦法已經被我們當中的很多人採納—買一個空氣濾清器。你也許已經發現，品質較好的空氣濾清器已經包含了一層活性碳過濾網以及一層HEPA過濾網，而這兩者都能除去PM2.5。室內的臭氧並沒有像PM2.5以及致癌的甲苯、甲醛氣體一樣危險。在中國新裝修的房子中，由於成本較低的家具、牆和地板的存在，類似的揮發性氣體的濃度經常居高不下。空氣濾清器的碳過濾層可以非常有效地去除臭氧和這些揮發性氣體。
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.
我最近有幾次嚴重的氣喘發作，雖然現在已經恢復，但是我對病情復發還是感到緊張。所以在我於這個夏天中慶幸尚未受到氣喘影響的同時，最近媒體關於臭氧的一系列報導也促使我有趕緊加快腳步來做臭氧的研究。我已經決定，唯一需要的改變就是要在臭氧濃度高的日子裡配戴含碳口罩（也同時含有N95材料）上下班。現在每當我打開顯示污染指數的手機程式的時候，除了看PM2.5的濃度以外，我也會看一眼臭氧的濃度。要不然，我在家裡其實已經夠安全的了，因為家裡的室內空氣濾清器就有活性碳和HEPA過濾層。
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.
在這個夏天裡，我也對兒童和得氣喘的病人格外關注，我要確保他們更加了解炎熱的天氣裡，臭氧會對他們的身體造成什麼樣的威脅。與往常一樣，在北京健康地生活需要普通常識與公共健康議題教育的結合。而對於防範臭氧來說，採用和針對PM2.5一樣的預防措施應該就可以讓大部分的人們保持健康了。
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