6 Common Hazardous Air Pollutants

Air pollutants are the unwanted substances that enter the atmosphere from various natural and anthropogenic sources. These solid, liquid or gaseous substances can be harmful to our health and surroundings.

They are divided into

Primary Pollutants

thick smoke from factories are the primary air pollutants

Pollutants that are emitted into the atmosphere directly are known as primary pollutants.

For example, carbon monoxide emitted from vehicle exhaust and sulfur dioxide from industries are primary pollutants.

Secondary Pollutants

Secondary pollutants do not enter the air directly but are rather formed in the atmosphere after the reactions between primary pollutants.

For example, harmful ozone is a secondary pollutant which is formed by the reaction between nitrogen oxides and volatile organic compounds.

The more widely found air pollutants have been classified as criteria pollutants worldwide. There are six common hazardous air pollutants that have been placed in this classification. They are as follows

Carbon monoxide(CO)

CO gas is tasteless and has no smell. So it is hard to detect and if enough concentration of the gas inhaled it becomes harmful to us.

  • Sources: CO is emitted into the atmosphere from vehicle exhaust, manufacturing processes in industries, tobacco smoke. Volcanoes and forest fires are natural sources of CO.
  • Effects: When high amount of CO enters our body, it reduces oxygen in our red blood cells which hamper the functions of organs. It causes clumsiness and increased fatigue. It also increases the risks of heart attack in people with history of heart problems. Infants and children are more affected.

carbon monoxide is one of the secondary pollutants in air


Lead is a heavy metal that occurs naturally in the earth surface. It is found in the air in a concentration not more than 0.1 microgram/m3.

  • Sources: There are many anthropogenic sources of lead such as mining, manufacturing of metal and concrete, leaded petrol, coal, ceramic products, car batteries and paints.
  • Effects: Lead is stored in our teeth and bones and can impart damage to our vital organs like brain, liver and kidneys.Exposure to varying level of lead can cause acute or chronic lead poisoning. It can have adverse effects to our immune system, cardiovascular system and reproductive system.

chart showing lead positining in human body

Sulfur dioxide

Sulfur dioxide reacts with other substances easily and forms harmful compounds like sulfuric acid and sulfurous acid. It has a pungent smell and almost 99% of the gas is emitted into the air by human activities.

  • Sources: The main sources are industrial activities such as production of electricity from fuels containing sulfur and fuel combustion in power plants. Other sources include extraction of metals and emissions from automobiles.
  • Effects: It irritates the airways and causes wheezing, coughing and difficulty in breathing. It may worsen asthma. It forms small particles after reaction with compounds and these particles can enter our airways causing adverse effects. It can aggravate respiratory diseases like emphysema and worsen heart diseases.

Harmful ozone

It is the ozone found near ground level that is harmful to us. It is formed by the reaction between nitrogen oxides and various volatile organic compounds when sunlight is present.

  • Sources: Major sources of the components of ozone are emissions from industries, exhaust from automobiles, chemical solvents, oil refining, petrochemicals, bush fires etc.
  • Effects: Exposure to enough levels cause irritation and pain in eyes, throat and nose. It reduces lung function and may cause respiratory ailments like difficulty in breathing, coughing. It can also aggravate existing problems like asthma.

Particulate matter

Particulate matter (PM) is a mix of extremely small fine or coarse particles. It is the main component of particle pollution which is one of the major type of air pollution.

  • Sources: Particulate matter can enter the atmosphere from several sources such as automobile exhaust, industrial emissions, burning of fossil fuels, tobacco smoke, forest fires, farming, road constructions etc.
  • Effects: Particles of small size can be inhaled into our lungs and can even enter the bloodstream. Particle pollution can aggravate diseases of lung and heart and respiratory ailments. It causes difficulty in breathing and irritation of respiratory tract.

Nitrogen dioxide (NO2)

NO2 is a highly reactive gas and is used as an indicator of nitrogen oxides. It is a component of photochemical smog that can be seen in most of our cities.

  • Sources: Burning of fossil fuels, automobile exhaust, metal refining, food processing, gas heaters etc. are common sources of nitrogen dioxide. Some of these gas are also formed by the action of lightening and chemical processes in plants and soil.
  • Effects: Exposure of different levels may cause airway inflammation, worsen asthma, bronchitis and emphysema, and aggravate diseases of the heart.