When the haze day comes, the street people are in a hurry to depart start wearing masks. However, wearing a mask, after all, a lot of the same, especially in the winter wearing eyes of people often be exhaled water fog. People usually wear masks when they go outdoors, such as home, or at the office. So does the mask really protect the human body? Do we have to wear a mask if we go out?
To understand these problems, the first need to understand the concept of exposure (exposure). Exposure is a measure of the health of a polluted environment to the human body. Its calculation formula is:
Exposure = contaminant concentration X exposure time
For PM2.5, the exposure of the unit is micrograms / cubic meter
This reason is easy to understand, is the impact of health is not just an indicator of the concentration of the haze, but also need to consider the time you are exposed in the haze. Even if the concentration is high, if the exposure time is short, the impact on health is not large. On the other hand, even if the concentration is not high, but the exposure time is very long, so it will be a great impact on health. Because the ultimate impact of health is the total amount of PM2.5 in the lungs of the lung. The assumption is that if you are running, you need to take a correction factor to calculate the total amount of exposure.
To decide whether to wear a mask, the first is to involve the air filter performance. A little bit of common sense people will know that ordinary gauze, cotton masks on the PM2.5 is basically no filtering effect. Small particles can drive straight in front of these masks, these masks in addition to let you feel a little better, without any help. These masks are not used, and we don't have a discussion here.
So what about PM2.5? The most common is the N95 mask. For this kind of performance is very good mask, the merchant will generally N95 as a big sign on the mask packaging. N95 Institute Health for Occupational Safety and (NIOSH) is a standard for air pollutants, which has 0.3 of the size of the air pollutant, and has a 95% filtration efficiency. Is only 5% per cent of the filter paper.
But the masks and air leakage coefficient. Face and face mask if you want to reach the filtering effect of its design, the general need to do FIT TESTING RESPIRATOR, which is the face and face of the mask test. I have never observed a person doing this test. It's usually a store of masks. So there will be leakage phenomenon. Fit depends on your face masks and the ordinary mask leakage rate between 20-30%.
If we consider the filtration efficiency of 95%, and 20% of the air leakage rate. Then the general N95 mask is the comprehensive efficiency of 76%.
The following figure is a scholar in Shanghai observed a continuous three days of fog and haze days, the change of indoor concentrations and outdoor concentrations of each hour. Before the rainy day, the situation is basically no haze, the third day of 200 micrograms / cubic meter or more of the heavy haze. In this picture, the house is still relatively good seal. Indoor haze increased with the increase of outdoor haze, but the concentration value is lower than the outside, and the peak has a relatively obvious lag.
According to this data, I'm here to estimate how a person's exposure to a day is.
We go out at 8 in the morning, at 9 to the office, and at 5 in the evening, at 6. Half the time in the subway or other closed vehicles. In the morning to go to work in the street on the walk for 30 minutes, the same time in the evening to go to work on the street is 30 minutes to consider. At noon in the outdoor activities for 30 minutes. Other time or in the office, or at home. This is a typical way of life for most people.
Then we look at the total number of times the total number of three hours of exposure and the proportion of total exposure.
Exposure values (micrograms / cubic meter) of total exposure
Home (late 8) 154253%
Office (9 a.m. - 104236% p.m.
Outdoors (not wearing a mask) 30313%
Total exposure value 2887
If all the time in the outdoor wear N95 masks, the new exposure value is how much
Exposure values (micrograms / cubic meter) of total exposure
Home (late 8) 154258%
Office (9 a.m. - 104238% p.m.
Outdoors (not wearing a mask) 723%
Total exposure value 2656
Can be seen, wearing a mask and not wearing a mask has little effect on the haze exposure, the total exposure value decreased from 2656 to 2887, the difference is only 8%. The main reason is because most of the exposure is indoor. Total exposure at home was 1542, office of exposure was 1042, which accounted for 53% of total exposure and 36%.
This calculation results in fact, and the concentration of the outside of the haze is not much, only depends on the concentration difference between indoor and outdoor, and you in the outdoor time. Even if the recent 450 micrograms / cubic meter of Beijing's heavy pollution, the conclusion is the same. Because indoor PM2.5 concentrations are always outside the 50-90%.
As mentioned earlier, most of the construction of indoor haze situation better than the outdoors. So if the indoor environment is not effective in improving the indoor environment, the short outdoor wear mask is not significant. Unless you're a long outdoor person. This is also a lot of people feel.

　　每当雾霾天来临的时候，大街上行色匆匆的人群纷纷开始带上了口罩。然而戴口罩毕竟有很多不变，尤其是冬天戴眼睛的人时常被呼出的水雾困扰。人们通常的习惯是在室外行走的时候戴上口罩，到了室内，比如回到家，或者到达办公室摘下口罩。那么口罩是否真的对人体产生有效的保护呢？我们外出出行是否必须戴口罩呢？

　　要想搞明白这些问题，首先需要明白的是暴露（exposure）这个概念。暴露是衡量一个污染环境对人体的健康程度影响的指标。 它的计算公式是：

　　暴露 = 污染物浓度 X 暴露时间

　　对于PM2.5而言，暴露的单位是微克/立方米•小时

　　这个道理很容易明白，就是对健康的影响不单单是雾霾浓度的指标，还需要考虑你暴露在雾霾中的时间。 即使浓度很高，如果暴露时间很短，对健康的影响并不大。另外一方面哪怕浓度不高，但是暴露时间很长，那么对健康影响也会很大。 因为最终影响健康的是吸入肺部的总的PM2.5的总量。上面的假设是人体的呼吸频率不变的情况下，如果你在奔跑状态，那么还需要乘上一个修正系数来计算暴露总量。

　　决定是否戴口罩，首先涉及的是口罩的空气过滤性能。稍有常识的人都会知道，普通的纱布、棉布口罩对PM2.5基本上是没有任何过滤效果的。小颗粒物在这些口罩面前可以长驱直入，这些口罩除了让你感觉好一点之外，没有任何帮助。 这些口罩是没有用的，不在我们这里的讨论之列。

　　那么那些针对PM2.5的口罩的呢？ 市面上最常见的就是N95口罩。 对于这类性能非常好的口罩，商家一般都会把N95作为大大的标志放在口罩包装上。 N95口罩是根据美国国家职业安全卫生研究所National Institute for Occupational Safety and Health (NIOSH) 的规范，对0.3微米以上粒径的空气污染物，有95%的过滤效率。就是只有百分之5%可以透过过滤纸。

　　但是口罩还有漏风系数。全脸和半脸的口罩如果想到达其设计的过滤效果，一般需要做RESPIRATOR FIT TESTING, 就是脸部和口罩的边界吻合测试。普通人群我从来没有观察到有人做这样的测试。一般就是商店口罩买一个戴上就行了。所以一般会有漏风现象。取决于口罩和你的脸的吻合度，普通口罩漏风率在20-30%之间。

　　如果考虑95%的过滤效率，和20%的漏风率。那么普通N95口罩的综合过滤效率就是76%。

　　下图是学者在上海观测到的一个连续三天雾霾天情况下，每个小时室内浓度和室外浓度的变化情况。之前是雨天，雾霾情况基本没有，第三天出现200微克/立方米以上的重雾霾。 在这张图里面，这个房子的密闭性还是比较好的。室内的雾霾随着室外的雾霾升高而升高，但是浓度值比室外要来得低，而且峰值有比较明显的滞后。

　　根据这个数据我来估算一下一个人一天的暴露情况是怎样的。

　　我们按照一个人早上8点出门，9点到达办公室，晚上5点下班，6点到家。路上一半的时间在地铁或者其他封闭交通工具里。早上上班的外出街上的步行时间为30分钟，晚上下班同样街上的步行时间是30分钟来考虑。中午在室外活动30分钟。其他时间或者在办公室，或者在家里。 这在大多数人是比较典型的生活方式。

　　那么我们来看一下三段时间的总雾霾暴露值是多少和占总暴露的比重。

  暴露值 （微克/立方米•小时） 占总暴露比重

居家（晚6点-早8点） 1542 53%

办公室（早9点-晚5点 1042 36%

户外（不戴口罩） 303 13%

总暴露值 2887

　　如果在户外的全部时间戴了N95的口罩，新的暴露值是多少

  暴露值 （微克/立方米•小时） 占总暴露比重

居家（晚6点-早8点） 1542 58%

办公室（早9点-晚5点 1042 38%

户外（不戴口罩） 72 3%

总暴露值 2656

　　可见，戴口罩和不戴口罩对雾霾暴露总值的影响不大，总暴露值从2887降低到2656，差别只有8%。主要原因是因为大部分的暴露都是室内。居家的总暴露是1542，办公室的暴露是1042，分别占总暴露的53%和36%.

　　这个计算结果其实和室外的雾霾浓度关系不大，只取决于室内外的浓度差别，和你在室外的时间。即使北京最近这样450微克/立方米的重度污染，结论也是一样的。因为室内PM2.5的浓度总是室外的50-90%之间。

　　如前文所述，大部分建筑室内雾霾情况比室外好不到哪里去。所以如果室内雾霾环境没有有效改善的情况下，短暂室外出行期间戴口罩意义是不大的。除非你是长时间室外工作的人。 这也是很多人觉得雾霾天，即使戴了口罩还是一样会喉咙发痒咳嗽一样。 但是如果你关注家庭环境，改善了室内雾霾到国际标准25微克/立方米，那么室外的这些暴露就变得异常重要了。

　　总结一下，回到一开始的问题，雾霾天是否需要戴口罩？ 纱线和棉布口罩是没有什么用的，即使N95的口罩，对你帮助杯水车薪。当然你可以说戴总比不戴要好。对于大部分早九晚五的工薪族而言，降低雾霾对你健康的危害，最重要的是你家里的室内环境，其次是办公环境，最后才是你在室外是否需要戴口罩的问题。戴口罩可以降低室外的暴露，但是如果室内的雾霾浓度值很高，室外的这点变化就变得相对不重要。 雾霾天如果只关心是否出行戴口罩，忘了室内雾霾的话，那么就是舍本取末。好比一个一天一包烟的人，其实他们什么口罩都不需要戴。