What Happens When AQI Hits 999, Is There Any Measurement Beyond That? Explained

Last Updated:November 18, 2024, 17:37 IST

When the AQI reaches 999, it is considered an emergency situation. In most cases, the AQI never exceeds 500, even in places with severe pollution.

Vehicles ply on the road amid low-visibility due to smog, in New Delhi. (PTI Photo)

The Air Quality Index (AQI) readings reached 999 in several locations in Delhi and nearby areas at 1 PM on Monday, November 18, marking a critical and hazardous situation, as no AQI metre is designed to display a value above 999. But what does this mean for public health, and why does the AQI system stop at 999?

To understand the significance of an AQI of 999, it’s useful to look back at a historical event: the Great Smog of 1952 in London. During this devastating episode, thick fog mixed with air pollution, causing pollution levels to exceed 500. The fog was so dense that it blotted out daylight, while hospitals were overwhelmed with patients suffering from respiratory distress. Between 10,000 and 15,000 people are estimated to have died in just three days. Experts believe the unusually high levels of airborne lead and other pollutants were largely responsible for the catastrophe.

Why Does the AQI Scale Have a Limit?

The AQI, which measures the concentration of pollutants like particulate matter (PM2.5), nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), and ozone, typically has a maximum value of 500 in most countries. This upper limit is considered a threshold for extremely hazardous air quality, signalling immediate risk to health.

However, some regions, including China, have extended the AQI scale to 999 to account for extreme pollution levels. These higher readings reflect an urgent public health crisis, where the air quality is deemed dangerously hazardous, and immediate protective actions are necessary.

AQI 999: A Global Indicator of Crisis

When the AQI reaches 999, it is considered an emergency situation. In most cases, the AQI never exceeds 500, even in places with severe pollution. For example, in the United States, the AQI rarely goes beyond 500, which is recognised as the critical threshold for health emergencies.

In China, the AQI scale can rise to 999 during periods of prolonged, extreme pollution, which have been known to cause significant health problems. Meanwhile, in India, it is not uncommon for AQI levels to peak at 999 in winter months, particularly following Diwali, due to the widespread burning of firecrackers and agricultural stubble burning.

Why is There No Scale Beyond 999?

The lack of a scale beyond 999 reflects the reality that air quality at these levels is already so dangerous that it is not practical to differentiate further. When the AQI hits its maximum value, it signifies a health emergency. At such high levels, every part of the population, particularly vulnerable groups such as children, the elderly, and those with pre-existing health conditions, are at serious risk. Immediate action – such as staying indoors, using air purifiers, wearing masks, and avoiding strenuous activities – is critical.

When the Air Quality Index (AQI) hits the extreme value of 999, it signals a dangerously high level of air pollution that poses an urgent threat to public health. In such situations, immediate action is required to protect both human health and the environment. Below is a detailed breakdown of the impact of such extreme pollution levels and the steps that should be taken.

Health Implications for HumansGeneral Population Risks

Even individuals with no pre-existing health conditions can experience severe health problems, including:

Difficulty breathingSore throatBurning eyes

Exposure to this level of pollution increases the risk of serious cardiovascular and respiratory conditions, including heart attacks, strokes, and chronic breathing issues.

Vulnerable Groups

Certain groups are especially at risk, including:

ChildrenElderly individualsThose with pre-existing heart or lung conditions (e.g., asthma, chronic obstructive pulmonary disease or COPD)

For these vulnerable groups, prolonged exposure can lead to life-threatening complications.

Pregnant Women

Expectant mothers are also at risk, as exposure to polluted air can negatively impact both maternal health and fetal development, increasing the likelihood of complications during pregnancy.

2. Environmental Impact

Reduced Visibility

Extreme pollution levels often cause smog or haze, which can reduce visibility to just a few metres. This drastically affects road safety and increases the risk of traffic accidents.

Long-term Agricultural Damage

Air pollution at these levels can have devastating effects on crops, leading to long-term reductions in agricultural productivity.

Harm to Wildlife

Animals, too, are not immune. Wildlife that relies on clean air for survival can suffer as pollution diminishes their natural habitats and affects their health.

3. Causes of AQI 999

A variety of factors contribute to extreme pollution levels:

Heavy industrial emissionsMassive vehicular pollutionBurning of agricultural stubble or biomassStagnant weather conditions that trap pollutants in the air

These factors combine to elevate the AQI to extreme levels, which can persist for extended periods in some cases.

4. Immediate Steps to Take

Stay Indoors: The most effective way to protect yourself is by staying indoors, avoiding all outdoor activities, and limiting exposure to polluted air.Seal Windows and Doors: Keep all windows and doors sealed to prevent outdoor air from entering your living or working space. This helps reduce indoor pollution levels.Use Air Purifiers: If possible, use air purifiers equipped with HEPA filters to help remove harmful particles from the air inside your home or office.Wear Protective Gear: If leaving the house is unavoidable, wear a high-quality N95 or P100 mask to filter out harmful pollutants and protect your lungs.

5. Monitoring Your Health

Pay attention to any symptoms such as:

Persistent coughChest painDizziness or headaches

If these symptoms worsen, it’s important to consult a healthcare professional immediately. Long-term exposure to hazardous air quality can lead to chronic health issues.

6. What Should Authorities Do in Such a Situation?

In response to an AQI of 999, governments and local authorities should:

Declare a public health emergencyRestrict vehicular movement to reduce emissionsClose schools and non-essential businesses to limit exposureShut down industries that are significant contributors to air pollution

These actions can help to mitigate the immediate health risks posed by the extreme air pollution.

7. Serious Long-term Dangers

Prolonged exposure to such dangerous levels of air pollution can lead to:

Increased disease incidence, including respiratory and cardiovascular diseasesHigher mortality rates, particularly among the most vulnerable groups like the elderly and those with pre-existing health conditions.

What is the AQI and How Does It Work?

The Air Quality Index (AQI) is a numerical scale used to measure and communicate the quality of the air. It ranges from 0 to 500, with higher values indicating worse air quality. The AQI is calculated based on several pollutants in the air, providing a real-time measure of air pollution levels and their potential health risks.

AQI Categories:

The AQI is divided into six distinct categories:

0-50 – Good: Air quality is considered satisfactory and poses little or no risk.51-100 – Moderate: Air quality is acceptable; however, some pollutants may be a concern for sensitive individuals.101-200 – Unhealthy for Sensitive Groups: Members of sensitive groups (children, elderly, those with respiratory conditions) may experience health effects.201-300 – Unhealthy: Everyone may begin to experience health effects, and sensitive individuals may face more serious effects.301-400 – Very Unhealthy: Health alert; everyone may experience more severe health effects.401-500 – Hazardous: A public health emergency is declared, and everyone may experience significant health issues.

Pollution Factors Affecting AQI

The AQI is calculated based on the following eight pollutants:

PM10 (Particulate Matter 10)PM2.5 (Particulate Matter 2.5)NO2 (Nitrogen Dioxide)SO2 (Sulfur Dioxide)CO2 (Carbon Dioxide)O3 (Ozone)NH3 (Ammonia)Pb (Lead)

These pollutants are measured over a 24-hour period to determine the air quality index.

What Are PM2.5 and PM10?

PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometres or less. Due to their small size, these particles can penetrate deep into the lungs, causing significant harm, especially during smog episodes.

PM10 refers to particulate matter with a diameter of 10 micrometres or less, which includes larger particles such as dust, dirt, and ash from burning waste.

What Are the Major Pollutants?

NO2 (Nitrogen Dioxide): Produced by high-temperature combustion processes, such as car engines and power plants.SO2 (Sulfur Dioxide): Emitted by burning fossil fuels like coal and oil.CO2 (Carbon Dioxide): A greenhouse gas that is toxic in high concentrations and produced by the incomplete combustion of organic fuels.O3 (Ozone): Formed when sunlight reacts with pollutants like NO2 and volatile organic compounds.NH3 (Ammonia): Released by agricultural activities, including livestock farming and fertiliser use.

Health Risks of Air Pollution

Fine particles, especially PM2.5, can infiltrate the lungs and enter the bloodstream, exacerbating respiratory conditions and increasing the risk of lung cancer. Individuals exposed to high levels of pollution may experience eye irritation, sore throats, coughing, and difficulty breathing. Long-term exposure can lead to chronic conditions like asthma, cardiovascular diseases, and lung cancer.

Natural Sources of Pollution

While much of the pollution comes from human activity, natural sources also contribute:

Dust storms from arid areasMethane emissions from livestock digestionRadon from the earth’s crustWildfires that release large quantities of carbon and particulate matter

In conclusion, when AQI levels hit 999, immediate protective measures are essential. Whether through government action or personal vigilance, the key is to minimise exposure and protect health until conditions improve.

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News explainers What Happens When AQI Hits 999, Is There Any Measurement Beyond That? Explained