Air Quality Index Or AQI, What Does It Mean? | Indoor & Outdoor Pollution

The Air Quality Index or AQI is a numerical scale used for reporting daily changes in the quality of the air of your local area, with regard to human health and the environment.

AQI Air Quality Index

Air pollution can harm anyone, but it can be really unsafe and hazardous for a lot of individuals, including infants and children, elderly people (anyone above 60 years of age), people with respiratory or cardiovascular disorders, etc. Even healthy individuals who are frequently working or exercising outdoors are at greater risk of being harmed by the hazards of air pollution.

You may also like to read: Environmental Toxicology- Study of Toxicants present In the Atmosphere

The local air quality affects the way you live or breathe. It changes on a day-to-day basis or sometimes even on an hourly basis. Information about the changing air quality of a particular area is important for the people of that area to make sure they adopt certain lifestyle changes in accordance with the current air quality.

Today, getting information about your local air quality is as easy as looking for a weather update. But unfortunately, most of the population is unaware of it and how to interpret this information.

The Air Quality Index or AQI is a numerical scale used for reporting daily changes in the quality of the air of your local area, with regard to human health and the environment.

The daily results are used to convey to the general public about the level of air pollution in their areas and probably what health hazards could be caused in such condition of the air. AQI focuses on health effects you may experience within a few hours or days after breathing unhealthy air.

In more than 800 counties all over the world, air pollution levels are measured daily and categorized on a scale of 0 for perfect air, all the way up to 500, for air pollution levels that pose an immediate danger to the public using the Air Quality Index.

The AQI further divides air pollution levels into six categories, each of which has a name, an associated colour and a piece of advice to go along with it.

What does AQI Value Represent?

In India, the National Air Quality Index was launched by the Ministry of Environment, Forest and Climate Change. Central Pollution Control Board along with State Pollution Control Boards has been operating the National Air Monitoring Program (NAMP) covering 240 cities of the country.

These bodies collect data on the status of the air quality in voluminous amount. Thus, it became important that information on air quality is put up in the public domain in simple and linguistic terms that can be easily understood by a common person.

There are six AQI categories, namely, Good, Satisfactory, Moderately polluted, Poor, Very Poor, and Severe. 

The proposed Air Quality Index considers eight pollutantsPM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb for which short-term (up to 24-hourly averaging period) National Ambient Air Quality Standards are prescribed.

Each category corresponds to a different level of health concern for the people exposed:

  • Good: If the AQI value for a community is between 0 and 50, air quality is satisfactory and it poses little or no health risk to the members of this community.
  • Satisfactory: When AQI values are between 51 and 100, the air quality is acceptable; though, pollution in this range may pose a moderate health concern for a very small number of individuals. People who are unusually sensitive to ozone or particle pollution may experience respiratory or similar difficulties.
  • Moderately polluted or Unhealthy for Sensitive Groups: When Air Quality Index values are between 101 and 150, members of sensitive groups may experience implications for health, but the general public is unlikely to be affected. Sensitive groups- People with heart or lung disease, elderly people and children are considered sensitive and therefore at greater risk.
  • Poor or Unhealthy: Everyone may begin to experience effects on their health when AQI values are in the range of 151 and 200. Members of sensitive groups may experience more serious detrimental health impacts.
  • Very poor or Very Unhealthy: Air Quality Index values between 201 and 300 can trigger a health alert, meaning everybody may experience more serious health effects.
  • Severe or Hazardous: AQI values over 300 trigger health warnings of emergency kind of situations. The entire population is more likely to be affected by seriously injuring health effects.

AQI Air Quality Index

What Parameters Are Evaluated To Measure Air Quality Index?

The National Air Monitoring Program considers eight major pollutants for calculating the Air Quality Index, viz. carbon monoxide, nitrogen dioxide, ammonia, ozone, sulphur dioxide, lead, particulate matter 2.5 and 10.

  • Carbon Monoxide

Carbon monoxide is a colourless, odourless gas that can be toxic when inhaled in large quantities. It is formed both naturally and by anthropogenic activities when fuels containing carbon are burnt in low-oxygen conditions (incomplete combustion of fuel).

Inhalation of carbon monoxide at high concentrations can be fatal because it prevents the transport of oxygen in blood in the body. Releases from poorly maintained appliances in poorly ventilated spaces could result in concentrations high enough to cause death. Long-term exposure to lower concentrations could harm fetuses and cause neurological damage.

  • Nitrogen Dioxide

Nitrogen dioxide (NO2) is a foul-smelling gas. Nitrogen dioxide is an important air pollutant because it contributes to the formation of photochemical smog, which can have significant impacts on human health. Breathing air with a high concentration of NO2 can irritate airways in the human respiratory system.

Such exposures over short periods can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms. Longer exposures to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections.

  • Ammonia

Gaseous ammonia (NH3) is the most abundant alkaline gas in the atmosphere. The largest source of NH3 emissions is agriculture, including animal husbandry and ammonia-based fertilizer applications. Other sources include industrial processes and vehicular emissions. Recent studies have indicated that NH3 emissions have been increasing over the last few decades on a global scale.

This is a concern because NH3 plays a significant role in the formation of atmospheric particulate matter and degradation of visibility. Thus, the increase in NH3 emissions negatively influences environmental and public health as well as climate change.

The harm caused by ammonia in water bodies is more serious because it is very toxic to aquatic organisms. On a wider scale, ammonia plays a role in the transportation and enhanced deposition of acidic pollutants – resulting in acidification of ground and water bodies, which can harm plant and animal life. The exposure to high concentrations could cause irritation of the eyes, nose and throat.

  • Ozone

Ozone (O3) can be healthy or unhealthy depending upon where it is found in the atmosphere. Stratospheric ozone is good for living beings as it protects us from the harmful ultraviolet rays of the sun.

Ground-level ozone or tropospheric ozone is harmful as it acts as a pollutant and it can trigger a number of health problems, especially in children, elderly people and all those who have some respiratory and cardiovascular condition.

Ground-level ozone is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). These reactions take place when emissions by cars, power plants, industrial boilers, refineries, etc., get exposed to sunlight.

NOx+ VOCs+ Sunlight+ Heat  Ozone

Ozone affects sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas. Ozone reduces lung function and harms lung tissue. It can worsen bronchitis, emphysema, and asthma, leading to increased medical care.

  • Sulphur Dioxide

Sulphur dioxide (SO2) has a nasty, strong smell. It reacts easily with other substances to form harmful compounds, such as sulphuric acid, sulphurous acid and sulphate particles. The main source of sulfur dioxide in the air is industrial activity like the generation of electricity from coal, oil or gas that contains sulfur, vehicular emissions, etc.

SO2 can react with other compounds in the atmosphere to form fine particles that reduce visibility and cause haze. At high concentrations, SO2 can harm trees and plants by injuring foliage and decreasing their growth.  SO2 can contribute to acid rain which can harm various ecosystems.

Sulphur dioxide affects human health when it is breathed in. It irritates the nose, throat, and airways causing coughing, shortness of breath, or a tight feeling around the chest. The implications of sulphur dioxide are felt very quickly and usually, people start feeling the severe symptoms in 10 or 15 minutes after breathing it in.

  • Particulate Matter

Particulate matter (PM) refers to all the solids particles and liquid droplets suspended in the air. This comprises soot, smoke, dust, pollens, etc. These particles diverge significantly in their sizes, origin and composition.
Based on their sizes, particulate matter is categorized into two types:

  • Larger particles ranging from 5 to 10 µm (PM10– PM2.5). These form the coarser type of particulate matter.
  • Smaller ones with a size up to 2.5 µm (PM5). These form the finer type of particulate matter. The particles in the fine fraction which are smaller than 0.1 µm are called ultrafine particles.

These microscopic solids or liquid droplets are so small that they can be inhaled and can cause serious health problems. Some particles less than 10µm in diameter can get deep into your lungs and some may even get into your bloodstream. PM2.5, pose the greatest risk to health.

  • Lead

Major sources of lead in the atmosphere are lead ores, metal processing, waste incinerators and lead-acid battery manufacturers.

The maximum concentration of lead is found in the air in areas near the lead smelters. Depending on the level of exposure, lead can abnormally affect the nervous system, renal system and reproduction in humans.

It gets accumulated in the bones by being transported through the blood in the entire body. It may also cause cardiovascular diseases and behavioural problems in children and young adults.

How Is Air Quality Index Or AQI Calculated?

Based on the measured ambient air concentrations, corresponding standards and likely health impact also known as health breakpoint, a sub-index is calculated for each of the pollutants.

A sub-index is a linear function of concentration (e.g. the sub-index for PM2.5 will be 51 at concentration 31 µg/m3, 100 at concentration 60 µg/m3, and 75 at concentration of 45 µg/m3).

The formula for calculating a sub-index is as follows:

Sub Index for a pollutant =

(Upper limit of the previous AQI category to which the pollutant’s current reading would have fallen) +[(current reading – the upper limit of the previous reading category of the pollutant)* (width or interval of the AQI category for the current level of reading/width or interval of the current reading category of the pollutant)]

The Sub-indices for individual pollutants at a monitoring location is calculated using its 24-hourly average concentration value (8-hourly in case of CO and O3) and health breakpoint concentration range.

All the eight pollutants may not be monitored at all the locations. A web-based system is premeditated to provide Air Quality Index on a real-time basis. It is a programmed system that collects data from continuous monitoring stations without human intrusion and displays Air Quality Index based on running average values.

For manual monitoring systems, an Air Quality Index calculator is developed where the data is entered manually to get the AQI values.

Indoor Air Pollution

Indoor Air Quality (IAQ) refers to the air quality within and around buildings and structures, especially when it relates to the health and comfort of the inhabitants. Understanding and controlling common pollutants indoors can help reduce your risk of indoor health concerns.

Sources of Indoor Air Pollution include the following:

  • Fuel-burning and combustion appliances
  • Tobacco products
  • Building materials and furnishings as diverse as:
    • Deteriorated asbestos-containing insulation
    • Newly installed flooring, upholstery or carpet
    • Cabinetry or furniture made of certain pressed wood products
  • Products for household cleaning and maintenance, personal care, or hobbies
  • Central heating and cooling systems and humidification devices
  • Excess moisture
  • Outdoor sources such as:
    • Radon
    • Pesticides
    • Outdoor Air Pollution

Air pollution is something that affects everyone. It is maximum during the heat of the day, so planning your outdoor activities during the early mornings or late evenings might help to reduce exposure to the pollutants.

Antioxidant-rich foods like coloured fruits and vegetables can also help shield your body from the damaging effects of free radicals created by air pollution.

Indoor pollution also needs to be checked by regularly vacuuming the confined spaces. Cleaning the filters of air conditioners can help to regulate outdoor pollution to enter indoors.

Leave a comment