The AQI (Air Quality Index) is a comprehensive air quality indicator developed by Taiwan's Ministry of Environment based on US EPA standards.[1][2] It converts concentrations of six major pollutants into a unified scale of 0–500, allowing the public to quickly understand air quality conditions and recommended protective actions.
| AQI Range | Level | Health Effects | General Public | Sensitive Groups |
|---|---|---|---|---|
| 0 – 50 | Good | Air quality is satisfactory; pollutant concentrations are very low | Normal outdoor activities | Normal outdoor activities |
| 51 – 100 | Moderate | Acceptable; some pollutants may cause minor effects for extremely sensitive individuals | Normal activities; monitor air quality changes | Consider reducing prolonged strenuous outdoor exercise |
| 101 – 150 | Unhealthy for Sensitive Groups | Sensitive groups (asthma, heart disease, children, elderly) may experience worsening symptoms | Reduce prolonged strenuous outdoor exercise | Reduce outdoor activity; wear a mask if necessary |
| 151 – 200 | Unhealthy | Everyone may start to experience respiratory irritation and eye discomfort | Reduce outdoor activities; wear N95 mask when going out | Avoid all outdoor activity; stay indoors |
| 201 – 300 | Very Unhealthy | Health alert; everyone may experience significant discomfort | Avoid outdoor activity; close windows and use air purifier | Stay indoors; avoid any physical exertion |
| 301 – 500 | Hazardous | Emergency health warning; entire population faces serious health risk | Stop all outdoor activities; seal indoor environment | Emergency response; seek medical care if needed |
AQI is calculated from the concentrations of six key pollutants, taking the highest sub-index as the AQI value for that hour. Each pollutant differs in its sources, chemical properties, health hazards, and regulatory standards. A deeper understanding helps you interpret air quality data more accurately.
Particles with a diameter of less than 2.5 micrometers — about 1/28th the width of a human hair. Because of their tiny size, they can penetrate the alveoli and enter the bloodstream, reaching vital organs such as the heart and brain.
Major sources include vehicle exhaust, factory emissions, open burning, and secondary aerosols formed by the atmospheric reactions of SO2 and NOx. In Taiwan, PM2.5 is the primary culprit behind air quality deterioration in autumn and winter. Long-term exposure is closely linked to cardiovascular disease, lung cancer, and stroke; each 10 μg/m³ increase raises all-cause mortality by approximately 6%.[3]
Particles with a diameter of less than 10 micrometers, which primarily settle in the nose, throat, and bronchi of the upper respiratory tract. The coarse fraction (PM10-2.5, excluding PM2.5) mostly originates from mechanical abrasion processes.
Sources include road dust, construction sites, natural windblown sand, agricultural tilling, and industrial emissions. Short-term high-concentration exposure can worsen respiratory diseases such as asthma and allergic rhinitis, and may also carry heavy metals and bacteria. Taiwan occasionally experiences Mongolian dust storms in spring, causing PM10 to spike to several hundred μg/m³ in a short time.
Ground-level ozone is a classic "secondary pollutant" — it is not directly emitted but is formed through photochemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) under intense ultraviolet radiation.
Concentrations peak between 12:00 and 16:00 in summer, particularly in the Taipei Basin and Kaohsiung. Inhaling excessive amounts irritates respiratory mucous membranes, reduces lung function, and worsens asthma symptoms. Long-term exposure may lead to chronic obstructive pulmonary disease (COPD). Interestingly, heavily trafficked roads tend to have lower ozone levels because NO consumes O3; higher concentrations are found downwind in suburban areas.
A colorless, odorless toxic gas produced primarily by incomplete combustion of carbon-containing materials — car engines, gas water heaters, industrial boilers, cigarette smoke, and more. Because it cannot be detected by human senses, it is extremely dangerous.
CO's affinity for hemoglobin is 200–250 times greater than that of oxygen, forming carboxyhemoglobin (COHb) that causes tissue hypoxia. Mild poisoning causes headache, dizziness, and nausea; severe exposure can lead to loss of consciousness or death. Ambient CO levels are generally low, but busy roads, underground parking lots, and enclosed spaces require special attention.
A colorless gas with a pungent smell, mainly emitted from coal-fired power plants, oil refineries, sulfide ore smelting, and combustion of sulfur-containing fuels. It is one of the primary precursors of acid rain.
When SO2 enters the respiratory tract, it dissolves in moisture to form sulfurous acid, irritating the eyes, nasal passages, and airway mucous membranes, causing bronchospasm. Asthma patients are particularly sensitive and may experience symptoms at low concentrations. In the atmosphere, SO2 can be oxidized to sulfate aerosol, becoming an important component of PM2.5. In recent years, Taiwan's SO2 emissions have declined significantly due to coal reduction policies.
A reddish-brown toxic gas with a pungent odor, mainly produced by high-temperature combustion — vehicle engines (especially diesel), power plants, and industrial boilers. Urban NO2 concentrations are highly correlated with traffic volume.
NO2 can penetrate deep into the lungs, triggering inflammation and increasing the risk of respiratory infections. Children are more vulnerable due to their higher breathing rates. Research shows long-term exposure is associated with a 20–30% increase in childhood asthma incidence. NO2 is also a key precursor of ground-level ozone and secondary PM2.5, playing a central role in photochemical smog formation.
Taiwan is situated in the subtropics and surrounded by sea, so air quality varies significantly with the seasons, terrain, and meteorological conditions. Understanding seasonal trends helps you plan outdoor activities and health protection in advance.
The northeast monsoon weakens and continental cold air masses have less influence. Occasional cross-border dust storms occur (about 2–4 times per year), causing PM10 to spike above 200 μg/m³. Rainfall is sparse before the plum rain season, leaving central and southern Taiwan with poor air quality on some days. Conditions gradually improve from May as rainfall increases.
The southwest airflow brings abundant convective rain, providing good atmospheric dispersion and washout; PM2.5 concentrations drop to their annual low. However, intense ultraviolet light drives strong afternoon photochemical reactions, elevating O3 in basin areas like Taipei. Typhoons can also briefly affect air quality through associated pressure changes.
The prevailing northeast monsoon carries pollutants southward from eastern China. The Central Mountain Range blocks dispersion eastward, leaving the western coastal plains most exposed. The Kaohsiung–Pingtung region typically shows orange-level warnings first, with air quality deteriorating noticeably.
The worst air quality season of the year. Cold high-pressure systems drive temperature inversions, creating an extremely stable atmosphere unfavorable to dispersion. Industrial pollutants from eastern China are transported over long distances by the northeast monsoon, and combined with local emissions, PM2.5 frequently reaches hazardous levels in the western half of Taiwan.
Taiwan covers only about 36,000 km², yet air quality varies dramatically across regions due to the Central Mountain Range, basins, plains, and differences in industrial structure and population density.
Basin topography causes cold air to sink in winter, easily forming temperature inversions. Traffic emissions are the primary pollution source (motor vehicles account for 40%+ of local emissions), and afternoon ozone is elevated in summer. Paradoxically, the northeast monsoon actually helps disperse pollutants southward during that season. Overall air quality is moderate to good by Taiwan standards.
Hsinchu's famous "Windy City" effect provides excellent atmospheric dispersion, with an average annual wind speed above 4 m/s. The Hsinchu Science Park generates some volatile organic compound (VOC) emissions, but at a limited scale. Miaoli's Tongxiao area occasionally records elevated SO2 due to its thermal power plant. Overall air quality is good.
The Taichung thermal power plant (originally 10 coal-fired units totaling 5,500 MW) is one of Taiwan's largest stationary pollution sources and is currently undergoing a coal-to-gas transition — units 1 & 2 are scheduled for demolition by 2026 and will be replaced by new gas-fired units;[11] under weak winter wind conditions, its influence can extend tens of kilometers. Changhua's coastal industrial zone contributes considerable emissions. Mountainous areas in Nantou, such as Sun Moon Lake, enjoy relatively better air quality due to higher elevation.
The Mailiao Petrochemical Complex in Yunlin is among the top three largest stationary emission sources in Taiwan. Chiayi, situated at the heart of the Chianan Plain, experiences heavy pollutant accumulation in autumn and winter. Tainan faces both petrochemical industries (Rende, Yongkang) and heavy traffic pollution, with frequent orange-to-red warnings in winter. This region is also a hotspot for open-field agricultural waste burning.
The region with the most severe air quality challenges in Taiwan. Kaohsiung hosts the island's densest belt of heavy industries (steel, petrochemicals, shipbuilding, cement), with Siaogang, Linyuan, and Daliao as pollution hotspots. The prevailing winter wind field continuously pushes accumulated pollutants southward, trapping the Kaohsiung–Pingtung area in a chronic air quality deficit. Pingtung's Chaozhou Basin further amplifies the accumulation effect.
The Central Mountain Range — averaging 3,000 m in elevation — acts as a natural barrier that effectively blocks pollutants from the western half of Taiwan. Combined with low population and industrial density (Hualien-Taitung's population density is only 1/10 that of the west), eastern Taiwan consistently records the best air quality on the island, making it a true "last clean land."
Penghu is surrounded by sea with an annual average wind speed exceeding 6 m/s, giving it generally good air quality. Kinmen and Matsu are only 2–10 km from the Chinese mainland coast, making them highly susceptible to trans-boundary pollution in winter — PM2.5 can spike to more than twice the mainland Taiwan average. Lienchiang County (Matsu) has recorded AQI levels above 300 (Hazardous).
When AQI reaches the orange (101+) or red (151+) level, appropriate protective measures can significantly reduce both the acute and chronic harm caused by air pollution. The following recommendations apply to the general public and sensitive groups alike.
Ordinary cloth masks or surgical masks filter PM2.5 at only 30–50% efficiency. N95 (≥95% filtration) or KN95 masks are recommended. When wearing, ensure the nose clip seals tightly with no gaps at the sides. Masks with an activated carbon layer provide additional adsorption of organic gases and odors. Replace daily or according to recommended usage hours.
Keep doors and windows closed to reduce outdoor infiltration (indoor PM2.5 is typically 50–80% of outdoor levels). Use a HEPA air purifier (CADR ≥ room area in m² × 10). Avoid indoor incense burning, smoking, or using VOC-containing cleaners and sprays. Consider adding window sealing strips to improve airtightness. Replace filters regularly (every 3–6 months).
During exercise, breathing volume increases 10–20 times, proportionally raising pollutant intake. AQI < 50: any outdoor exercise is fine; 50–100: shorten high-intensity sessions; >100: move indoors. If you must run outside, choose early morning (5–7 am, when traffic pollution is lowest) or after rain, and stay at least 200 m from major roads.
Children (breathing rate twice that of adults), adults aged 65+, pregnant women, and those with asthma, COPD, or cardiovascular disease are high-risk groups. At AQI > 100, minimize outings and keep emergency medications on hand. Watch for early symptoms: persistent coughing, chest tightness, difficulty breathing, eye irritation, or palpitations — seek medical attention promptly if they occur.
Pollution exposure by commute mode: motorcycle > bicycle > bus > MRT > private car (interior recirculation). Motorcycle riders inhale 2–3 times more PM2.5 than passengers in a car. Recommendations: wear an N95 mask, avoid congested routes, choose less-trafficked alternative roads. Switching a car's air intake to interior recirculation mode can reduce external pollution by up to 80%.
Consume foods rich in antioxidants: vitamin C (citrus fruits, kiwi), vitamin E (nuts, avocado), omega-3 fatty acids (deep-sea fish, flaxseed), and lycopene (tomatoes). Drink plenty of water to keep airway mucous membranes moist and promote ciliary clearance of particles. Cruciferous vegetables (broccoli, cabbage) contain sulfur compounds that support the body's detoxification pathways.
The World Health Organization (WHO) has classified outdoor air pollution as a Group 1 carcinogen.[6] Approximately 4.2 million people worldwide die prematurely each year from outdoor air pollution.[7] The following summarizes its effects on different body systems.
Long-term exposure to PM2.5 leads to a continuous decline in lung function, increasing the risk of chronic obstructive pulmonary disease (COPD), lung cancer, bronchitis, and childhood asthma. Each 10 μg/m³ increase in PM2.5 raises lung cancer risk by approximately 8% and all-cause mortality by 6%. Lung cancer has been Taiwan's leading cancer killer for several consecutive years, with the proportion of non-smokers among lung cancer patients rising annually.
Fine particles entering the bloodstream trigger systemic inflammation and oxidative stress, accelerating atherosclerosis and raising blood pressure. Long-term exposure significantly increases the risk of myocardial infarction, stroke, arrhythmia, and heart failure. A meta-analysis covering 650,000 individuals found that each 10 μg/m³ increase in PM2.5 raises cardiovascular mortality risk by 11%.
Ultrafine particles (diameter < 0.1 μm) can travel directly into the brain via the olfactory nerve and are associated with increased incidence of Alzheimer's and Parkinson's diseases. Children with prolonged exposure to high-pollution environments may experience impaired brain white matter development, reduced cognitive function, and lower academic performance. Recent research also links air pollution to elevated risks of depression and anxiety disorders.
Prenatal exposure to high concentrations of PM2.5 is associated with adverse outcomes including preterm birth (risk ↑ 18%), low birth weight, gestational hypertension, and preeclampsia. Air pollution particles have been proven to cross the placental barrier, affecting fetal lung and immune system development. Some studies suggest a possible increased risk of autism spectrum disorder in children.
Air quality regulatory standards vary greatly between countries. The WHO significantly tightened its guidelines in 2021, reflecting the latest epidemiological evidence. Taiwan's 2025 revised standards are the strictest in Asia, but remain more lenient than the latest WHO guidelines.
| Pollutant | Taiwan Standard[4] | US EPA[10] | EU | WHO 2021 Guideline[5] |
|---|---|---|---|---|
| PM2.5 (Annual mean) | 12 μg/m³ | 9 μg/m³ | 25 μg/m³ | 5 μg/m³ |
| PM2.5 (Daily mean) | 30 μg/m³ | 35 μg/m³ | — | 15 μg/m³ |
| PM10 (Annual mean) | 30 μg/m³ | — | 40 μg/m³ | 15 μg/m³ |
| O3 (8-hour) | 60 ppb | 70 ppb | 60 ppb | 100 μg/m³ |
| NO2 (Annual mean) | 30 ppb | 53 ppb | 40 μg/m³ | 10 μg/m³ |
| SO2 (Annual mean) | 30 ppb | — | — | — (daily mean 40 μg/m³ only) |
This site's content is based on the following authoritative sources and peer-reviewed literature. All real-time monitoring data comes from Taiwan's MOENV API.
The most common questions about air quality in Taiwan, covering everything from AQI basics to everyday protection strategies.