Dust particles with diameter ranging from 0.1 to 500 ㎛ are characterized according to their size and weight and divided into two major categories: Settleable particles (Dust fall) and Total Suspended Particles (TSP). Dust fall refers to particles that are heavy and settle on the ground by gravity, while TSP refers to particles that are fine and light and suspended in ambient air. TSP is the term for a total amount of suspended dust in the air. Major sources of TSP comprise combustion · incineration · heat treatment processes which use fossil fuel, firing · drying · reversible· desulfurization processes for petrochemical product manufacturing, unpaved roads, vacant lands, coal manufacturing facilities, etc.

Dust is suspended in the air to settle on plants and block pores in leaves and in particular, a layer of dust on leaves blocks sunlight and reduces the plant’s anabolism, respiration, and transpiration, resulting in negative effects on plant growth. In addition, inhalable dust gets into the human body and settle deeply into the breathing passages. Some dust particles could be removed by coughing, sneezing, ciliary movement, but others may settle and accumulate on pulmonary alveolus, causing damage to the human body.

Particulate matter (PM) is defined as fine particle dust whose diameter is below 10 ㎛ (1 ㎛= 1/1000 ㎜), which is hardly visible and much finer than a strand of human hair with diameter between 50 and 70 ㎛. To be more specific, PM is the term for a mixture of solid particles and liquid droplets in the air. PM is divided into two major categories according to its size: PM₁₀ and PM_2.5. PM₁₀ refers to a particle with diameter less than or equal to 10 ㎛, while PM_2.5 refers to a particle with diameter less than or equal to 2.5 ㎛. Sources of PM can be natural and anthropogenic. Natural sources consist of soil particles, sea salt, pollen, fungi, bacteria, volcanic ash, etc. However, PM is mainly produced by human activities such as transport including cars, ships, industry (e.g. factories, powerplants), agriculture and everyday activities including using boilers.

PM, one of harmful air pollutants, causes visibility degradation over certain regions by scattering light and interferes with metabolism function of plants by accumulating on plant leaves. PM also corrodes buildings and sculptures by accumulating on them. The bigger problems is PM negatively affects the human body as it is attributed to respiratory and lung diseases as well as skin and eye diseases. Unlike other dust, PM is accumulated in the human body without being filtered by the nose, mouth, or bronchus. PM_2.5 may get deep into the lung and bloodstream and cause cardiovascular diseases.

Sulfur Oxides (SOx) refer to compounds of sulfur and oxygen molecules, including SO, SO₂ and SO₃. SO₂ can be easily oxidized to SO₃ and turn into sulfate or react to produce H₂SO₄ . Major sources of SO are made up of combustion facilities for heating · power generation · industrial purposes, melting and smelting facilities for steelmaking, oil refinery, chemical fertilizer manufacturing facilities, fossil fuel combustion facilities, etc.

SO₂ in ambient air is fairly water‑soluble, making it easy to be absorbed into upper respiratory tract and irritate mucous membranes of the eye, nose, and other human body parts. Chronic exposure to SOx may cause pulmonary diseases such as bronchitis, asthma, pulmonary emphysema, pneumonia, etc.

Nitrogen Oxides (NOx) refer to a binary compound of nitrogen and oxygen, including NO, NO₂, N₂O, N₂O₃, N₂O₄, and N₂O₅. Among them, NOx is generally meant to refer to NO and NO₂. Major sources of NOx consist of chemical material manufacturing facilities, steelmaking machinery that uses nitric acid, combustion facilities that use fossil fuel, etc.

Exposure of human body to NO₂ may cause chronic bronchitis, pneumonia, pulmonary hemorrhage, pulmonary edema, etc. Exposure of plants to NOx destroys plant cells, causing formation of brown or dark brown spots on plant leaves.

Volatile Organic Compounds (VOCs) have high vapor pressure and relatively easily evaporate to the atmosphere. VOCs refer to organic compounds that exist as gases, or non‑methane carbon compounds. VOCs create photochemical reactions by absorbing solar‑radiation energy, similar to how NOx reacts. It also produces photochemical oxidants such as ozone and Peroxyacetyl nitrate (PAN), resulting in photochemical smog. Major sources of VOCs consist of facilities that use organic solvents, printing facilities, petrochemicals manufacturing facilities, oil refinery, oil storage tank, gas station, laundry, etc. Vehicles (e.g. cars, trains, ships, and aircraft) are also responsible for VOCs emissions.

VOCs are precursors to photochemical oxidants and known as one of carcinogens as exposure of human skin and respiratory organs to VOCs may disrupt the nervous system. Toxic chemicals including aromatic compounds (organic compounds with benzene ring), halogenide halide and hydrocarbon in the air may increase their risk of getting cancer.

Ammonia (NH₃) is a compound of nitrogen and hydrogen. It is a colorless, irritating and corrosive gas with alkaline properties. NH₃ reacts with SOx and NOx to produce secondary air pollutants such as ammonium sulfate and ammonium nitrate. Major sources of NH₃ are manufacturing facilities, livestock, fertilizer, human waste, etc.

Exposure of human body to high concentrations of atmospheric NH₃ may cause cough, headache, vomiting, and diarrhea. Because of its corrosivity, low concentrations of liquid NH₃ may cause severe damage to tissues of upper airways and high concentrations of liquid NH₃ may also result in skin flare and severe inflammation.

Carbon monoxide (CO) is an odorless, colorless gas. It is produced by incomplete fuel combustion under the condition where oxygen is lacking. CO emits mainly from the transport sector, especially cars. Its sources can be divided into two: natural (e.g. wildfire) and anthropogenic (e.g. industrial process, fuel combustion, cooking, smoking, and heating)

Long‑term inhalation of CO makes hemoglobin transform into Carboxyhemoglobin (CO‑Hb) and weakens oxygen transport function, resulting in lack of oxygen, which may have a negative impact on the central nervous system and the brain. Notably, since high concentration of CO is so toxic that it may cause severe health damage to even a healthy person.

Black Carbon (BC) is a particle with diameter less than or equal to 2.5 ㎛, which result from the incomplete combustion of fuel (coal, oil, and gas), wildfire, or biomass burning. In general, car exhaust and black smoke coming out of stacks contain BC. Long‑term inhalation of BC may impair cognitive capability.

BC absorbs solar‑radiation energy, contributing to global warming. In fact, CO₂, the biggest contributor of global warming, is responsible for 40% of raising the earth’s temperature, followed by BC, the second biggest one, responsible for 18%.