Saturday, September 21, 2019

NEA vs Aqicn.org data on haze

On 19/9/19 in Today paper "NEA addresses questions over the veracity of PSI, as alternative air quality reading emerge".  Same old story over the years every time when haze descends upon Singapore - NEA desperately defending their 24hrs average data.

NEA Dr Khairunnisa Yuhya even accused aqicn.org haze reading as being 'incorrect' compare to NEA 24 hr PSI reading. Aqicn.org is not incorrect but more accurate as it is instantcast reading.  Besides their data is obtained from NEA hourly reading, so how can it be incorrect ?  If NEA converts its 1 hr PM2.5 reading in ug/m3 into PSI, its reading will be very close to those of aqicn.org too.

Why does NEA not convert its hourly reading into PSI ? Their 24 hr average PSI reading is actually  converted from ug/m3. The only reason for not doing so for the 1 hr reading is that they don't want the public have easy comparison.  Most folks only remember the number above '100' PSI means unhealthy. But they may not be aware that 56 ug/m3 in the 1 hr reading is equivalent to above 100 PSI. The descriptive 'elevated' in 1 hr reading has the same meaning as 'unhealthy' in 24 hr averaged data. (note: as safe exposure limit for PM2.5 is base on 24 hrs, for conservative safety reason, we can base 1 hr safe expose limit on the 24 hr recommendation ie 56 ug/m3 'elevated' being the start of unhealthy range)

NEA don't seems to understand that for public safety - its 24 hr average reading is not helpful at all.  As an analogy, if there is a chemical gas leak in a factory - workers will be evacuated base on real time reading. Emergency response team will put on protective mask base on real time reading.  This is the same for environmental pollution - public should base their decision on real time reading and NOT averaged down 24 hrs data.

Environment pollution 24 hour data is used to gauge exposure health issues that may arises. Public decision to wear face mask or to stay indoor or not - should be based on real time reading (1 hr data).  It is NOT safe to base decision on 24 hour averaged down reading !

Aqicn.org under their FAQ explains in details how they derive their reading.  They also explained why they think 24 hr averaging is a "very bad idea" as air pollution is dynamic and no one can wait for 24 hrs before knowing that the air quality is good.

NEA should consider using Nowcast data formula suggested in aqicn.org website which is more reflective of haze suitition.

The NowCast is computed from the most recent 12 hours of PM monitoring data, but the NowCast weights the most recent hours of data more heavily than an ordinary 12-hour average when pollutant levels are changing. The NowCast is used in lieu of a 24-hour average PM concentration in the calculation of the AQI until an entire calendar day of hourly concentrations has been monitored." 

NEA should stop being so defensive and get their priority right by making the haze reading/descriptive easy for public to understand.  





Saturday, September 14, 2019

TV haze broadcast of 1 hr reading is NOT in PSI

Many are not aware that there is a difference in PSI reading and ug/m3 (microgram per meter cube). This is because the health warning levels are different.

NEA 24 hour average reading is in PSI. However NEA 1 hour average reading is in ug/m3.

S'pore official health advisory is based on 24 hours average PSI reading. For our personal health concern, we should base our activities on 1 hour real time reading ie to stay indoor or not, when to use face mask.

Table 1 : Health advisory by NEA in PSI for 24 hour averaged reading:
0 -50        Good
51-100     Moderate
101-200   Unhealthy
201-300   Very Unhealthy
above 300 Hazardous

Table 2 : Health advisory by NEA in ug/m3 for 1 hour averaged reading,
0-55        Normal
56-150    Elevated 
151-250  High
above 250 Very High

Table 3 below is the previous descriptive reference from NEA website from past years which is no longer found in their current site :

Table 3:
Index Category           PSI            24hrPM2.5 ug/m3
Good                           0-50          0-12
Moderate                   51-100      13-55
Unhealthy                101-200    56-150  
Very Unhealthy        201-300     151-250
Hazardous             above 301     above 251

As there is no safety limit given for 1 hr reading, we could take the unhealthy range level as 56 ug/m3 and above.

For the haze reading broadcast over TV provided by NEA, the 24 hr data is in PSI. But the 1 hr data does not have a unit a measurement attached to it - this may mislead the public into thinking it is in PSI. We can see from Table 3 - unhealthy level is at 56 ug/m3.

NEA is not helping the public as they could have easily converted the reading into PSI which the public is familiar with. They could have also used the same descriptive for both 24 and 1 hour data. Public may not understand that 'elevated' is 'unhealthy'.  


NOTE : 

1) US EPA adopts 35 ug/m3 as unhealthy limit for 24 hrs PM2.5 which is a more stringent standard than S'pore.

2) For real time 1 hourly data on haze please refer to : aqicn.org  - which provides air pollution Air Quality Index (AQI) for more than 60 countries in the world. AQI descriptive :

0-50                 Good
51-100             Moderate
101-150           Unhealthy for sensitive groups
151-200           Unhealthy
201-300           Very Unhealthy
301-500           Hazardous