Air Quality Index (AQI) Information
Air Quality Index (AQI)
The AQI presents air-quality data in a format that is easy for the public to understand. It informs people of the air-pollution situation in each area and whether it affects health. A single AQI value represents the concentrations of six air pollutants:
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PM2.5 (fine particulate matter ≤ 2.5 µm)
Particles with aerodynamic diameter ≤ 2.5 µm produced by combustion sources such as vehicles, agricultural burning, wildfires, and industrial processes. They can reach the alveoli and cause respiratory and pulmonary diseases. High or prolonged exposure can lead to accumulation in lung tissue, reduced lung function, bronchitis, and asthma.
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PM10 (particulate matter ≤ 10 µm)
Particles with aerodynamic diameter ≤ 10 µm from fuel combustion, open burning, industrial processes, crushing/grinding, and construction. They can accumulate in the respiratory tract and impact health.
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Ozone (O₃)
A colorless to pale-blue gas with a pungent smell, slightly water-soluble. Tropospheric ozone (near the ground) is the air pollutant, formed by reactions of nitrogen oxides and volatile organic compounds under sunlight. Health effects include eye and respiratory irritation and reduced lung function, especially in children, the elderly, and those with chronic lung disease.
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Carbon Monoxide (CO)
A colorless, odorless, tasteless gas generated by incomplete combustion of carbon-based fuels. It binds to hemoglobin about 200–250 times more strongly than oxygen, forming carboxyhemoglobin (COHb) and reducing oxygen transport to body tissues, causing fatigue and additional strain on the heart.
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Nitrogen Dioxide (NO₂)
A nearly colorless, odorless gas, slightly soluble in water, occurring naturally and from human activities such as fuel combustion and certain industries. It can affect vision and aggravate asthma or other respiratory conditions.
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Sulfur Dioxide (SO₂)
A colorless (sometimes faintly yellow) gas with noticeable taste/odor at high levels. Emitted naturally and from combustion of sulfur-containing fuels. It dissolves well in water and can form fine particles with other pollutants. Health impacts include eye, skin, and respiratory irritation; long-term exposure may lead to chronic bronchitis.
Thailand’s AQI has five levels, from 0 to 201 and above. Each level uses a color to indicate the degree of health impact (Table 1).
AQI = 100 corresponds to the national ambient air-quality standard. Values above 100 mean the pollutant concentration exceeds the standard and air quality begins to impact public health.
Table 1: Thailand AQI Criteria
| AQI | Color | Meaning | Description |
|---|---|---|---|
| 0 - 25 |
|
Very Good | Everyone can carry on daily activities as usual. |
| 26 - 50 |
|
Good |
General public: Normal outdoor activities. Sensitive groups: Monitor symptoms such as frequent cough, breathing difficulty, shortness of breath, wheezing, chest tightness/pain, palpitations, nausea, unusual fatigue, or dizziness. |
| 51 - 100 |
|
Moderate |
General public: Reduce duration of strenuous outdoor
activities. Sensitive groups:
|
| 101 - 200 |
|
Beginning to Affect Health |
General public: – Use protection (e.g., PM2.5 mask) when outdoors. – Limit time for strenuous outdoor activities. – Watch for symptoms such as cough, breathing difficulty, eye irritation. Sensitive groups: – Use protection (e.g., PM2.5 mask) when outdoors. – Avoid strenuous outdoor activities. – Follow medical advice; see a doctor promptly if symptoms occur. |
| ≥ 201 |
|
Health Impact |
Everyone: – Avoid outdoor activities. – If outdoor activity is necessary, always use protection (PM2.5 mask). – Seek medical care if symptoms occur. – People with underlying diseases should stay in clean-air areas, prepare necessary medicines/equipment, and strictly follow medical advice. |
The AQI is calculated from measured pollutant concentrations. The concentration ranges corresponding to AQI levels are shown in Table 2. Within an interval, the AQI is computed by a linear equation as follows.
Among the calculated sub-indices, the highest sub-index determines the
AQI at that time.
Air-quality sub-index
Measured pollutant concentration
Lower and upper bounds of the concentration interval containing X
Lower and upper bounds of the AQI interval corresponding to X
Table 2: Pollutant concentrations equivalent to AQI ranges
| AQI | 24-hour average | 8-hour average | 1-hour average | |||
|---|---|---|---|---|---|---|
| PM2.5 (µg/m³) | PM10 (µg/m³) | O₃ (ppb) | CO (ppm) | NO₂ (ppb) | SO₂ (ppb) | |
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Averaging periods and units used in the calculation
- PM2.5 — 24-hour running average : micrograms per cubic meter (µg/m³)
- PM10 — 24-hour running average : micrograms per cubic meter (µg/m³)
- O₃ — 8-hour running average : parts per billion (ppb)
- CO — 8-hour running average : parts per million (ppm)
- NO₂ — 1-hour average : parts per billion (ppb)
- SO₂ — 1-hour average : parts per billion (ppb)
Note: The Pollution Control Department (PCD) has issued additional approved measurement methods beyond the NEB methods.
- ppm = parts per million
- mg/m³ = milligrams per cubic meter
- µg/m³ = micrograms per cubic meter
- – = not specified
Adapted from
(1) NEB Announcement No. 10 (1995): National Ambient Air Quality Standards
(general).
(2) NEB Announcement No. 21 (2001): SO₂ 1-hour standard (general).
(3) NEB Announcement No. 24 (2004): National Ambient Air Quality Standards
(general).
(4) NEB Announcement No. 28 (2007): National Ambient Air Quality Standards
(general).
(5) NEB Announcement No. 33 (2009): NO₂ standard (general).
(6) NEB Announcement (2022): PM2.5 standards (Royal Gazette Vol. 139, Special 163 Ng, 8 Jul
2022).
(7) NEB Announcement (2017): CS₂ standard (general).
(8) NEB Announcement No. 30 (2007): VOCs 1-year standard (general).
(9) PCD Announcement (2019): Approved instruments and averaging methods for gases and
particulates (general).