A haze of numbers? Here's your guide to decoding AQI, PM2.5 and PSI

As air quality in Singapore deteriorated over Monday (Sept 14), with all regions across Singapore recording 24-hour Pollutant Standard Index (PSI) readings in the unhealthy range , some netizens have wondered why official readings appear different from what they are seeing on other air quality platforms.

One common question raised has been why the Republic does not use the Air Quality Index (AQI) that is publicly available from open sources such as the World Air Quality Index website (aqicn.org).

The project, founded by a team of contributors in Beijing, computes an air quality index using the United States Environmental Protection Agency's (US EPA) Air Quality Index (AQI) reporting system.

Meanwhile, in Singapore, the National Environment Agency (NEA) presents two key air quality measures on its haze microsite, namely the 24-hour Pollutant Standards Index (PSI) and the one-hour PM2.5 reading.

Taking to its social media on Tuesday, NEA said that its air quality data is measured by pollutant analysers, which continuously measure the level of particulate matter, including PM10 and PM2.5, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide.

It added that the analysers, which are placed across the five regions of Singapore, are highly specialised equipment.

"(These analysers) are based on internationally recognised methods, including instruments that adopt US EPA Federal Reference/Equivalent Methods (FRM/FEM). This is to ensure measurements are reliable and scientifically robust," it said.

Cutting through the 'haze' of information

There are currently no international guidelines or legally-binding standards on how air quality indices should be computed, with countries adopting different index systems based on their own needs and circumstances.

The AQI presented on aqicn.org draws on the 24-hour average of PM2.5 concentration data that is posted every hour by the NEA.

While PM2.5 measures the concentration of fine particulate matter in the air, AQI is a calculated index that converts pollutant data, including PM2.5, ozone and nitrogen dioxide, into a single colour-coded health risk score.

AQI may also vary between countries as different governments and entities may use different formulas to calculate the ranges of their respective AQI categories.

Meanwhile, PM2.5 refers to tiny particles that can enter the lungs and bloodstream and is considered one of the key pollutants affecting human health during haze episodes.

However, such raw numbers do not immediately tell a lay person what precautions to take.

This is why NEA groups PM2.5 readings into four bands: normal (0-55 µg/m3), elevated (56 to 150), high (151 to 250), and very high (251 and above).

It also "translates" what this means into an outdoor activities planning guide for healthy and vulnerable groups planning to conduct or participate in such activities immediately.

The 24-hour PSI is then used as the Republic's official national reporting system for air quality, providing a broader picture of pollution levels over the previous 24 hours.

It is accompanied with a corresponding health advisory, which can then be used to plan for next day activities, including whether schools should continue to remain open.

If you are wondering why the duration of 24 hours is used, that is because scientific and epidemiological studies on the health effects of exposure to particulate matter have been premised on this duration.

Poor visibility, smell and air quality

On its frequently asked questions section, NEA also explained why poor visibility does not mean that the air is more polluted.

It stated that a significant factor affecting visibility is the amount of water vapour in the air.

"Water droplets scatter light, resulting in lower visibility. Certain air pollutant particles attract water molecules, which results in larger particles that scatter more light. Such water-bound particles cause poor visibility to persist for a longer time.

"Thus, on days with smoke haze, high humidity or rain, visibility can be much lower."

As for the burning smell, NEA explained that the smell is usually, but not always, accompanied by an increase in one-hour PM2.5 concentration readings or PSI.

It pointed out that vegetation and peatland fires can also generate a complex mixture of gases which cause an acrid burning smell.

"These gases causing smell are different from the air pollutants monitored for air quality. Sometimes, the occurrence of some fires in your vicinity due to localised burning activities, may also contribute to the burning smell, and not transboundary haze," NEA said.

Which reading should I pay attention to?

If you're deciding whether to exercise, work or spend time outdoors right now, the one-hour PM2.5 reading is usually the most relevant measure.

PM2.5 reflects current air quality conditions, making it a useful indicator when planning immediate outdoor activities.

Members of the public should note that the health impact of haze depends on their health status, as well as the duration and intensity of outdoor activity.

For the general population, symptoms that may arise from short-term exposure include eye, nose or throat irritation.

When PM2.5 readings are in the elevated band, members of the public should reduce strenuous outdoor activity for the next hour.

Vulnerable persons such as the elderly, pregnant women, children, and persons with chronic lung or heart disease should avoid strenuous outdoor activity.

However, if readings hit the high-band, all healthy persons should avoid strenuous outdoor activity for the next hour, while vulnerable persons should avoid all outdoor activity.

When should I wear an N95 mask?

N95 masks are not required for short periods of exposure, such as commuting from home to work or school, or while in an indoor environment.

When the forecasted air quality is in the very unhealthy range (PSI >300), a healthy individual who has to be outdoors for several hours may wear an N95 mask to reduce exposure.

The Ministry of Health said on its website that the best form of protection for people with chronic lung or heart disease, elderly, and pregnant women is to avoid or minimise outdoor activity when the forecasted air quality is unhealthy (PSI > 200).

However, if they have to be outdoors for several hours, they may reduce exposure by wearing an N95 mask.

The elderly, pregnant women and people with severe lung or heart problems who have difficulty breathing at rest or on exertion should consult their doctor as to whether they should use an N95 mask.

Normal surgical masks, such as those worn during Covid-19, are not effective in filtering fine particles that are 2.5 microns or less in width.

周一（9月14日），新加坡空气质量恶化，全新加坡所有地区的24小时污染物标准指数（PSI）读数均处于不健康范围，一些网民对此感到疑惑，为什么官方读数与他们在其他空气质量平台上看到的数据有所不同。

经常有人问，为什么共和国不使用空气质量指数（AQI），而空气质量指数可以从世界空气质量指数网站（aqicn.org）等开放来源公开获取。

该项目由北京的一群贡献者创立，利用美国环境保护署（US EPA）的空气质量指数（AQI）报告系统计算空气质量指数。

与此同时，在新加坡，国家环境局 (NEA) 在其雾霾微网站上展示了两项关键的空气质量指标，即 24 小时污染物标准指数 (PSI) 和 1 小时 PM2.5 读数。

周二，国家环境局在其社交媒体上表示，其空气质量数据由污染物分析仪测量，这些分析仪持续测量颗粒物（包括 PM10 和 PM2.5）、二氧化硫、二氧化氮、臭氧和一氧化碳的水平。

声明还补充说，这些分析仪分布在新加坡的五个地区，都是高度专业化的设备。

“（这些分析仪）基于国际公认的方法，包括采用美国环保署联邦参考/等效方法（FRM/FEM）的仪器。这是为了确保测量结果可靠且科学严谨，”声明中写道。

拨开信息的“迷雾”

目前尚无关于如何计算空气质量指数的国际准则或具有法律约束力的标准，各国根据自身需求和情况采用不同的指数体系。

aqicn.org 上显示的空气质量指数 (AQI) 是根据国家环境局 (NEA) 每小时发布的 24 小时 PM2.5 浓度数据平均值计算得出的。

PM2.5 衡量空气中细颗粒物的浓度，而 AQI 是一个计算指数，它将包括 PM2.5、臭氧和二氧化氮在内的污染物数据转换为单一颜色编码的健康风险评分。

不同国家的空气质量指数 (AQI) 也可能有所不同，因为不同的政府和机构可能会使用不同的公式来计算各自 AQI 类别的范围。

同时，PM2.5 指的是能够进入肺部和血液的微小颗粒，被认为是雾霾期间影响人类健康的主要污染物之一。

然而，这些原始数字并不能立即告诉普通人应该采取哪些预防措施。

这就是为什么新加坡国家环境局将 PM2.5 读数分为四个等级：正常（0-55 µg/m3）、升高（56 至 150）、高（151 至 250）和非常高（251 及以上）。

它还“转化”了这一含义，为计划立即开展或参与此类活动的健康和弱势群体提供户外活动规划指南。

然后，24 小时 PSI 被用作共和国官方的国家空气质量报告系统，提供过去 24 小时污染水平的更广泛情况。

同时还附有相应的健康建议，可用于规划第二天的活动，包括学校是否应该继续开放。

如果你想知道为什么使用 24 小时的时长，那是因为有关接触颗粒物对健康影响的科学和流行病学研究都是基于这个时长进行的。

能见度差、气味难闻、空气质量差

新加坡国家环境局在其常见问题解答部分还解释了为什么能见度差并不意味着空气污染更严重。

报告指出，影响能见度的一个重要因素是空气中的水蒸气含量。

水滴会散射光线，导致能见度降低。某些空气污染物颗粒会吸引水分子，形成更大的颗粒，从而散射更多光线。这些与水结合的颗粒会导致能见度低的情况持续更长时间。

“因此，在烟雾弥漫、湿度高或下雨的日子里，能见度可能会大大降低。”

至于烧焦的气味，NEA解释说，这种气味通常（但不总是）伴随着一小时PM2.5浓度读数或PSI的增加。

报告指出，植被和泥炭地火灾还会产生复杂的混合气体，从而产生刺鼻的燃烧气味。

“这些引起异味的气体与空气质量监测中检测到的空气污染物不同。有时，您附近因局部燃烧活动而发生的火灾也可能导致烧焦的气味，而不是跨境烟霾，”国家环境局表示。

我应该重点关注哪些阅读材料？

如果你正在决定是锻炼、工作还是在户外消磨时间，那么一小时 PM2.5 读数通常是最相关的衡量标准。

PM2.5 反映当前的空气质量状况，因此是计划近期户外活动时的一个有用指标。

公众应注意，雾霾对健康的影响取决于他们的健康状况，以及户外活动的持续时间和强度。

对于一般人群而言，短期接触可能出现的症状包括眼睛、鼻子或喉咙刺激。

当 PM2.5 读数处于较高水平时，公众应在接下来的一个小时内减少剧烈的户外活动。

老年人、孕妇、儿童以及患有慢性肺病或心脏病等弱势群体应避免剧烈的户外活动。

但是，如果读数达到高值，所有健康的人在接下来的一个小时内都应该避免剧烈的户外活动，而易感人群应该避免所有户外活动。

我应该在什么情况下佩戴N95口罩？

在短时间内暴露于病毒环境中，例如从家到工作地点或学校的通勤途中，或者在室内环境中，不需要佩戴 N95 口罩。

当预测空气质量处于非常不健康范围（PSI > 300）时，健康的人如果要在户外待几个小时，可以佩戴 N95 口罩来减少暴露。

卫生部在其网站上表示，对于患有慢性肺病或心脏病的人、老年人和孕妇来说，最好的保护方式是在预测空气质量不健康（PSI > 200）时避免或尽量减少户外活动。

但是，如果他们必须在户外待上几个小时，佩戴 N95 口罩可以减少接触病毒的风险。

老年人、孕妇以及患有严重肺部或心脏疾病，在休息或运动时呼吸困难的人，应咨询医生是否应该使用 N95 口罩。

普通外科口罩，例如新冠肺炎疫情期间佩戴的口罩，不能有效过滤宽度为 2.5 微米或更小的细小颗粒。