DECISION

PROMULGATING A GUIDE TO CALCULATION AND PUBLISHING OF VIETNAM AIR QUALITY INDEX (VN_AQI)

DIRECTOR GENERAL OF VIETNAM ENVIRONMENT ADMINISTRATION

Pursuant to the Law on environmental protection dated January 01, 2015;

Pursuant to the Government's Decree No. 19/2015/ND-CP dated February 14, 2015 providing guidelines for the implementation of the Law on environmental protection;

Pursuant to the Government's Decree No. 40/2019/ND-CP dated May 13, 2019 on amendments to Decrees providing guidelines for the implementation of the Law on environmental protection;

Pursuant to the Decision No. 15/2018/QD-TTg dated March 12, 2018 of the Prime Minister defining functions, tasks, powers and organizational structure of Vietnam Environment Administration affiliated to the Ministry of Natural Resources and Environment;

At the request of the Director of Environmental Quality Management Department and the Director of Northern Center for Environmental Monitoring,

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Article 1. A Guide to calculation and publishing of Vietnam Air Quality Index (VN_AQI) is promulgated together with this Decision.

Article 2. This Decision comes into force from the day on which it is promulgated and supersedes the Decision No. 878/QD-TCMT dated July 01, 2011 of Director General of Vietnam Environment Administration promulgating the Manual for calculation of air quality index.

Article 3. Chief of Office of Vietnam Environment Administration, Director of Environmental Quality Management Department, Director of Northern Center for Environmental Monitoring, heads of other agencies/units affiliated to Vietnam Environment Administration, and relevant organizations and individuals shall implement this Decision./.

DIRECTOR GENERAL




Nguyen Van Tai

GUIDE

TO CALCULATION AND PUBLISHING OF VIETNAM AIR QUALITY INDEX (VN_AQI)
(Enclosed with Decision No. 1459/QD-TCMT dated November 12, 2019 of Vietnam Environment Administration)

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1.1. Objectives

This document provides instructions on calculation and publishing of Vietnam air quality index (VN_AQI) based on monitoring data obtained from automatic and continuous air quality monitoring stations.  

1.2. Recommended application

Environmental agencies, organizations and individuals engaging in the establishment and publishing of environmental quality data and information in Vietnam are recommended to follow instructions in this document. 

1.3. Definitions 

1.3.1. Vietnam air quality index (VN_AQI) is an index which is calculated based on monitoring data about air pollutants in Vietnam, used to report air quality and helps understand the impact of air quality on human health, and measured on a scale.  

1.3.2. Daily AQI (AQI^d) is the AQI value that represents the air quality in a day.

1.3.3. Hourly AQI (AQI^h) is the AQI value that represents the air quality in an hour.

1.3.4. Individual AQI (AQI_x) is the calculated AQI value of each monitoring pollutant. AQI_x includes the individual AQI per day (AQI_x^d) and the individual AQI per hour (AQI_x^h).

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1.3.6. Air quality monitoring station refers to a fixed-site or mobile motoring station which is used to automatically and continuously measure air quality parameters.   

1.4. Assessment of AQI values

AQI is calculated on a scale (ranges of AQI values) with specific icons and colors indicating the levels of air pollution and health risks. To be specific: 

Table 1: AQI values and corresponding air pollution levels

AQI values

Air pollution levels

Colors

Colors in RBG

0 - 50

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Green

0;228;0

51 - 100

Moderate

Yellow

255;255;0

101 - 150

Unhealthy for sensitive groups

Orange

...

...

...

151 - 200

Unhealthy

Red

255;0;0

201 - 300

Very unhealthy

Purple

143;63;151

301-500

...

...

...

Maroon

126;0;35

2. Instructions on calculation of AQI

2.1. Requirements to be satisfied when calculating VN_AQI

a. Data used for calculating VN_AQI

- Monitoring equipment must have its quality controlled and calibrated in accordance with law regulations.

- Monitoring data shall be processed before being used for computation of AQI values in order to ensure that all outliers are removed and the used monitoring data shall comply with the process of data quality assurance and control.

b. Methods of using data for calculating VN_AQI

- VN_AQI values to be calculated include hourly AQI and daily AQI.  Data used for calculating VN_AQI values shall be average 1-hour, 8-hour and 24-hour monitoring values.

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- At each monitoring station, AQI_x value shall be calculated for each monitoring pollutant. The final AQI value shall be the highest of AQI_x values of each pollutant.

- Pollutants used in calculating VN_AQI include SO2, CO, NO2, O3, PM10, and PM2.5.

- The calculation of VN_AQI must have at least one of two mandatory pollutants, PM10 and PM2.5. 

2.2. Calculation of VN_AQI values

2.2.1. Calculation of hourly AQI values (AQI^h)

Hourly AQI values shall be calculated with average 1-hour monitoring values.  

a. Calculation of Nowcast values for PM2.5 and PM10

Let c₁, c₂, …c₁₂  represent 12 average hourly monitoring values (c₁ represents the most recent hourly value, and c₁₂ represents the hourly average for 12 hours before present).

The weight factor is calculated as follows:        

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Cmax represents the highest value of 12 average hourly monitoring values.

If                   Let        

If                Let w=w*

Where the Nowcast value is given by:

Where  the Nowcast value is given by: 

Notes:

- Nowcast value is only calculated when at least 2 of 3 values c₁, c₂, c₃ are available. Conversely, it is considered as “missing data” (Nowcast value cannot be calculated).

- If c_i value is not available, w^i-1 = 0.

b. Calculation of AQI^h for each individual pollutant (AQIx)

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Where:

AQI_x: The AQI value for pollutant x

BPi: The lower concentration limit of the value of monitored pollutant as prescribed in Table 2, corresponding to level i

BPi+1: The upper concentration limit of the value of monitored pollutant as prescribed in Table 2, corresponding to level i+1

Ii: The AQI value at level i as given in the table below corresponding to BPi

Ii+1: The AQI value at level i+1 as given in the table below corresponding to BPi+1

Cx: The average hourly monitoring value of pollutant x. Nowcastx: Nowcast value calculated in section a.

Table 2: BP_i values for pollutants

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Ii

Bpi given for each pollutant (Unit: µg/m³)

O₃(1-hour)

O₃(8-hour)

CO

SO₂

NO₂

PM₁₀

PM_2.5

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0

0

0

0

0

0

0

0

2

...

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160

100

10.000

125

100

50

25

3

100

...

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...

120

30.000

350

200

150

50

4

150

300

...

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...

45.000

550

700

250

80

5

200

400

210

...

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800

1.200

350

150

6

300

800

400

90.000

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2.350

420

250

7

400

1.000

-

120.000

2.100

...

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500

350

8

500

≥1.200

-

≥150.000

≥2.630

≥3.850

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≥500

Notes:

- AQI^h for O₃ shall be calculated with 1-hour BPi value for O₃

- AQI^d for O₃ shall be calculated with 1-hour and 8-hour BPi values for O3

c. Overall hourly AQI

After having calculated AQI_x for each pollutant, the highest AQI value among all of values of monitored pollutants shall be the overall hourly AQI.

AQIh = max(AQIx)

Note: The AQI shall be rounded to the nearest integer.

2.2.2. Calculation of daily AQI values (AQI^d)

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- Values for PM2.5 and PM10: 24-hour averages.

- O3 value: The daily maximum 1-hour average and the daily maximum 8-hour average.

- Values for SO2, NO2 and CO: daily maximum 1-hour averages.

a. Determining daily maximum 1-hour average, daily maximum 8-hour average and 24-hour average

- The daily maximum 1-hour average is the highest of 1-hour average values.

- The daily maximum 8-hour average is the highest of 8-hour average values. The 8-hour average value is the arithmetic mean of 1-hour average values continuously measured during 8 hours.

E.g. Calculating the daily maximum 8-hour average with the following table of 1-hour average values for O₃:

Time

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19:00

20:00

21:00

22:00

23:00

00:00

1:00

2:00

3:00

...

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15,7

14,2

17,7

18,9

19,3

15,7

19,7

22,6

...

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29,0

Time

4:00

5:00

6:00

7:00

8:00

9:00

10:00

...

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...

12:00

13:00

14:00

O₃(1-hour average)

31,9

25,3

34,7

35,2

41,6

...

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49,2

55,8

69,6

78,3

91,5

Time

15:00

16:00

17:00

...

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...

19:00

20:00

21:00

22:00

23:00

0:00

O₃(1-hour average)

97,7

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71,8

43,5

34,3

21,5

20,5

19,4

20,4

21,3

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Time

1:00

2:00

3:00

4:00

5:00

6:00

7:00

8:00

...

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...

10:00

11:00

12:00

O₃(8-hour average)

18,0

19,4

21,2

23,0

23,8

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28,2

30,9

33,8

36,6

39,9

44,7

Time

13:00

14:00

...

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...

16:00

17:00

18:00

19:00

20:00

21:00

22:00

23:00

0:00

...

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...

51,3

58,4

66,2

71,1

74,4

73,7

71,0

65,0

57,8

...

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39,1

31,6

The daily maximum 8-hour average is 74,4.

- The 24-hour average is the arithmetic mean of 1-hour average values continuously measured within a day (from 01:00 to 00:00 of the following day).

b. Calculation of AQI^d for each individual pollutant (AQI_x)

The daily AQI for SO2, CO,  NO2, O3, PM10, PM2.5 are calculated by adopting the following formula 1:

Where:

BP_i and I_i values shall follow table 2.

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BPi: The lower concentration limit of the value of monitored pollutant as prescribed in Table 2, corresponding to level i

BPi+1: The upper concentration limit of the value of monitored pollutant as prescribed in Table 2, corresponding to level i+1

Ii: The AQI value at level i as given in the table below corresponding to BPi

Ii+1: The AQI value at level i+1 as given in the table below corresponding to BPi+1

Cx shall be provided for as follows:

- For PM2.5 and PM10: Cx values are 24-hour average values.

- For O₃: Cx values are daily maximum 1-hour and 8-hour averages.

Notes: The AQI for O₃ shall not be calculated if the daily maximum 8-hour average is higher than 400 µg/m³ (In this case, the AQI shall be calculated with the daily maximum 1-hour average).

- For SO2, NO2 and CO: Cx value is the daily maximum 1-hour average.

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After having calculated daily AQI_x for each pollutant, the highest AQI value among all of values of monitored pollutants shall be the overall daily AQI.

AQId = max(AQIx)

Note: The daily AQI shall be rounded to the nearest integer.

2.3. Example calculation

a. Calculation of Nowcast value

With PM_2.5 monitoring data given in the following table:

09:00

10:00

11:00

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...

13:00

14:00

15:00

16:00

17:00

18:00

19:00

20:00

26,9

...

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...

20,5

23,5

19,5

16,5

19,0

16,5

20,3

22,4

19,6

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Define:

Because w*=0,61 > 0,5, w=w*=0,61

b. Calculation of hourly AQI

With hourly monitoring averages given in the following table:

O3 (µg/m³)

NO2 (µg/m³)

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136,1

118,7

20,3

Define AQI values for pollutants:

Thus, AQIh = 60.

c. Calculation of daily AQI

With monitoring data given in the following table:

O3 (µg/m³)

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NO2 (µg/m³)

PM2.5 (µg/m³)

The daily maximum 8-hour average

The daily maximum 1-hour average

The daily maximum 1-hour average

24-hour average

89,3

114,6

130,8

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Define AQI values for pollutants:

Thus, AQId = 110.

3. Instructions on publishing of AQI

Information relating to VN_AQI shall be published when data and calculations comply with provisions in Section 2 hereof.

Information relating to VN_AQI shall be made available to the public by means of mass media such as press, radio, television, electronic display boards, websites and mobile applications.

Table 3: Requirements for VN_AQI-related information

Information to be published

In the press

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Television

Electronic display boards

Websites

Mobile app

Color for the corresponding AQI level

X

-

X

X

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X

Details of the monitoring station

Abbreviated name of the monitoring station

Abbreviated name of the monitoring station

Abbreviated name of the monitoring station

Abbreviated name of the monitoring station

Site, co-ordinate

Site, co-ordinate

Monitored pollutants used for calculating AQI.

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Pollutant with the highest AQIx

Pollutant with the highest AQIx

Pollutant with the highest AQIx

All pollutants.  Pollutant with the highest AQIx must be specified.

All pollutants.  Pollutant with the highest AQIx must be specified.

Recommended precautions, health warnings

X

X

X

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X

X

Method for calculating VN_AQI

Brief description according to name of the issuing authority of AQI formula

Brief description according to name of the issuing authority of AQI formula

Brief description according to name of the issuing authority of AQI formula

Brief description according to name of the issuing authority of AQI formula

Detailed description of calculation method

Detailed description of calculation method

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X

X

X

X

X

X

Notes: (x): compulsory; (-):optional

Table 4: VN_AQI levels and health implications

AQI values

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0 – 50 (Good)

Air quality is considered satisfactory, and air pollution poses no health risk.

51 – 100 (Moderate)

Air quality is acceptable. However, sensitive groups (such as the elderly, children and those with respiratory or cardiovascular problems, etc.) may suffer from a moderate health concern.

101 – 150 (Unhealthy for sensitive groups)

Members of sensitive groups may experience health effects. The general public is not likely to be affected.

151 – 200 (Unhealthy)

Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.

201 – 300 (Very unhealthy)

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301-500 (Hazardous)

Health warnings of emergency conditions: the entire population is more likely to be affected.

Table 5: VN_AQI levels and recommended activities

AQI values

Recommended activities for the general public

Recommended activities for sensitive groups

0 – 50 (Good)

Enjoy your usual outdoor activities

Enjoy your usual outdoor activities

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Enjoy your usual outdoor activities

Members of sensitive groups are recommended to watch for symptoms such as coughing or shortness of breath, but can still be active outside.

101 – 150 (Unhealthy for sensitive groups)

Everyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing outdoor activities.

For students, it’s ok to be active outside, but they are recommended to reduce prolonged strenuous exercise.

Members of sensitive groups are recommended to reduce physical exertion and outdoor activities.

People with asthma may need to use their reliever inhalers more often.

151 – 200 (Unhealthy)

Everyone should reduce outdoor physical exertion, avoid prolonged strenuous exercise, and take more breaks during outdoor activities.

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201 – 300 (Very unhealthy)

Everyone should reduce outdoor activities and move all activities indoors.

If it is necessary to go out, please wear a qualified mask.

Members of sensitive groups should stay indoors and reduce physical exertion.

301-500 (Hazardous)

Everyone should stay indoors and keep doors and windows closed. If it is necessary to go out, please wear a qualified mask.

Quyết định 1459/QĐ-TCMT năm 2019 về hướng dẫn kỹ thuật tính toán và công bố chỉ số chất lượng không khí Việt Nam (VN_AQI) do Tổng cục trưởng Tổng cục Môi trường ban hành