Pollutant Standards Index

The Pollutant Standards Index, or PSI, provides a uniform system of measuring pollution levels for the major air pollutants. It is based on a scale devised by the United States Environmental Protection Agency (USEPA) to provide a way for broadcasts and newspapers to report air quality on a daily basis.

The PSI is reported as a number on a scale of 0 to 500 and is the air quality indicator. These index figures enable the public to determine whether the air pollution levels in a particular location are good, unhealthy, hazardous or worse. The PSI is used in a number of countries including the United States and Singapore. However, since 1999, the United States EPA has replaced the Pollution Standards Index (PSI) with the Air Quality Index (AQI) to incorporate new PM2.5 and ozone standards.

PSI in Singapore

Singapore has yet to replace the PSI with the Air Quality Index. Instead, it publishes the PSI and the PM2.5 Concentration separately. Although PSI is derived by averaging data collected for the past 24 hours, Singapore publishes its PSI based on data from the past 3 hours instead. This 3-hour PSI is unique to Singapore and was introduced in 1997 to provide additional air quality information which would better reflect a more current air quality situation.

Singapore has been occasionally hit by smoke haze from forest fires in nearby Sumatra, Indonesia, brought over by wind. These forest fires have been attributed to the slash-and-burn method favoured by several farmers to clear their land, as opposed to a more expensive and inconvenient mechanical approach using excavators and bulldozers. In June 2013, severe haze hit Singapore, pushing the nation's PSI into Hazardous levels for the first time in its history. Presently, the highest 3-hour PSI reading on record in Singapore is 401 on 21 June 2013 at 12 pm (GMT+8).

As the haze worsens, some Singaporeans have questioned the accuracy of the PSI readings, arguing that it does not tally with the situation outside their windows. Those who were more cynical even wondered whether data was doctored so as not to cause public panic, fear and alarm. Dispelling the doubts, Dr Vivian Balakrishnan, Singapore's Minister for the Environment and Water Resources said that data for all PSI readings will also be published on the Government’s data.gov.sg website “so there is no question about data abstraction or amputation”. Singapore's Prime Minister Mr Lee Hsien Loong added that there might seem to be discrepancies in the published readings when the haze outside one’s window “looks terrible”, but this was because the figure represented the average reading over three hours, and so could be better or worse than what is experienced on the ground.

The dire haze situation that has plagued Singapore in June 2013 is causing persistent PSI readings in the Unhealthy to Hazardous range. The colour codes used in the 3-hour PSI readings table below are unofficial and are used for purposes of clarity.

This is a list of PSI readings since June 17 that fall within the "Hazardous" range (ie. above 300). The readings in bold indicate that the particular PSI reading is or was a new record.

1. 401 (June 21 - 12 pm)
   
2. 400 (June 21 - 11 am)
   
3. 371 (June 20 - 1 pm)
   
4. 367 (June 21 - 10 am)
   
5. 360 (June 21 - 1 pm)
   
6. 355 (June 20 - 2 pm)
   
7. 326 (June 22 - 10 am)
   
8. 323 (June 22 - 9 am)
   
9. 322 (June 22 - 11 am)
   
10. 321 (June 19 - 10 pm)
    
11. 319 (June 22 - 12 pm)
    
12. 312 (June 20 - 3 pm)
    
13. 310 (June 20 - 6 pm)

3-hour PSI readings in June 2013

Date/Time	12am	1am	2am	3am	4am	5am	6am	7am	8am	9am	10am	11am	12pm	1pm	2pm	3pm	4pm	5pm	6pm	7pm	8pm	9pm	10pm	11pm
17	—	—	—	—	—	—	—	56	56	55	57	64	80	95	100	105	111	110	111	117	140	152	155	150
18	145	—	—	—	—	—	109	106	108	115	121	123	114	104	95	90	85	81	82	88	97	108	122	133
19	134	—	—	—	—	—	77	78	80	84	91	103	124	152	170	172	158	146	144	161	190	290	321	282
20	218	195	—	—	—	—	137	128	122	131	153	198	299	371	355	312	253	268	310	292	231	197	231	250
21	210	173	143	119	104	96	94	111	158	256	367	400	401	360	245	168	145	143	139	135	137	142	153	168
22	180	183	180	179	177	180	190	231	292	323	326	322	319	263	178	122	85	73	73	77	82	87	N/A	N/A
0–50 Good 51–100 Moderate 101–200 Unhealthy 201–300 Very unhealthy 301- Hazardous

The following PSI table is grouped by index values and descriptors, explaining the effects of the various bracketed levels as coloured above, according to the National Environment Agency (NEA).

PSI	Descriptor	General Health Effects
0–50	Good	None
51–100	Moderate	Few or none for the general population
101–200	Unhealthy	Mild aggravation of symptoms among susceptible persons i.e. those with underlying conditions such as chronic heart or lung ailments; transient symptoms of irritation e.g. eye irritation, sneezing or coughing in some of the healthy population.
201–300	Very Unhealthy	Moderate aggravation of symptoms and decreased tolerance in persons with heart or lung disease; more widespread symptoms of transient irritation in the healthy population.
301–	Hazardous	Early onset of certain diseases in addition to significant aggravation of symptoms in susceptible persons; and decreased exercise tolerance in healthy persons.

Note: This chart reflects the guidelines used in Singapore and may differ from other countries. Health advisories are based on the USEPA’s guidelines. Only the 24-hour PSI value and not the 3-hour PSI value is correlated to the health effects outlined in NEA’s advisories. NEA’s health advisories are determined by the worse of the 24-hour PSI and 24-hour PM2.5 value.

The NEA publishes the current readings on the last 24-hour cycle of both the PSI and the PM_2.5 readings.

The following chart is taken from page 4 of the United States' Environmental Protection Agency's (EPA) Fact Sheet of Standards and Air Quality Index to which the discrepant standards of the US' Air Quality Index (AQI) and Singapore's PSI can be seen.

Compare with an example PSI publication screenshot taken from the Singapore National Environmental Agency's (NEA) website on 5am, 21st June:

File:PSI Reading 5am 21 Jun.jpg

PSI Reading 5am 21 Jun

While the US' AQI would register "Very Unhealthy to Hazardous" given PM2.5 readings of 214-260 μg/m3, the Singapore National Environmental Agency's use of the PSI classifies the standard as being "Unhealthy".

PSI (API) in Malaysia

The air quality in Malaysia is reported as the API or Air Pollution Index, which is based closely on the PSI. Four of the index's pollutant components (i.e., carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide) are reported in ppmv but PM₁₀ particulate matter is reported in μg/m³.

Unlike the American AQI, the index number can exceed 500. Above 500, a state of emergency is declared in the reporting area. Usually, this means that non-essential government services are suspended, and all ports in the affected area closed. There may also be a prohibition on private sector commercial and industrial activities in the reporting area excluding the food sector.

AQHI in Canada

In Canada air pollution and the health risks associated are measured with the The Air Quality Health Index or (AQHI). It is a health protection tool used to make decisions to reduce short-term exposure to air pollution by adjusting activity levels during increased levels of air pollution.

The Air Quality Health Index or "AQHI" is a federal program jointly coordinated by Health Canada and Environment Canada. However, the AQHI program would not be possible without the commitment and support of the provinces, municipalities and NGOs. From air quality monitoring to health risk communication and community engagement, local partners are responsible for the vast majority of work related to AQHI implementation. The AQHI provides a number from 1 to 10+ to indicate the level of health risk associated with local air quality. Occasionally, when the amount of air pollution is abnormally high, the number may exceed 10. The AQHI provides a local air quality current value as well as a local air quality maximums forecast for today, tonight and tomorrow and provides associated health advice.

1

2

3

4

5

6

7

8

9

10

+

Risk:

Low (1-3)

Moderate (4-6)

High (7-10)

Very high (above 10)

As it is now known that even low levels of air pollution can trigger discomfort for the sensitive population, the index has been developed as a continuum: The higher the number, the greater the health risk and need to take precautions. The index describes the level of health risk associated with this number as ‘low’, ‘moderate’, ‘high’ or ‘very high’, and suggests steps that can be taken to reduce exposure.

Health Risk	Air Quality Health Index	Health Messages
		At Risk population	*General Population
Low	1-3	Enjoy your usual outdoor activities.	Ideal air quality for outdoor activities
Moderate	4-6	Consider reducing or rescheduling strenuous activities outdoors if you are experiencing symptoms.	No need to modify your usual outdoor activities unless you experience symptoms such as coughing and throat irritation.
High	7-10	Reduce or reschedule strenuous activities outdoors. Children and the elderly should also take it easy.	Consider reducing or rescheduling strenuous activities outdoors if you experience symptoms such as coughing and throat irritation.
Very high	Above 10	Avoid strenuous activities outdoors. Children and the elderly should also avoid outdoor physical exertion.	Reduce or reschedule strenuous activities outdoors, especially if you experience symptoms such as coughing and throat irritation.

It is measured based on the observed relationship of Nitrogen Dioxide (NO₂), ground-level Ozone (O₃) and particulate matter (PM_2.5) with mortality from an analysis of several Canadian cities. Significantly, all three of these pollutants can pose health risks, even at low levels of exposure, especially among those with pre-existing health problems.

When developing the AQHI, Health Canada’s original analysis of health effects included five major air pollutants: particulate matter, ozone, and nitrogen dioxide (NO2), as well as sulphur dioxide (SO₂), and carbon monoxide (CO). The latter two pollutants provided little information in predicting health effects and were removed from the AQHI formulation.

The AQHI does not measure the effects of odour, pollen, dust, heat or humidity.

See also

• Haze
• Air Quality Index
• Air Quality Health Index
• Air Pollution Index
• 2006 Southeast Asian haze
• 1997 Southeast Asian haze
• 2013 Southeast Asian haze

External links

• FAQ on the haze
• Past and present PSI readings in Singapore published by the NEA