Deadly Air: The Worst Pollution Crises

Air pollution is a major threat to public health and is responsible for more than 10% of all deaths worldwide, with 99% of the global population breathing unhealthy air. The World Health Organization (WHO) has deemed air pollution as the greatest environmental health risk, with short-term exposure leading to difficulty breathing, chest pain, and irritation of the eyes, nose, and throat, and long-term exposure causing lung tissue damage, cancer, and heart disease. While air pollution is a problem everywhere, the worst air quality is found in mountainous terrain, with Nepal topping the list due to its location between China and India, creating natural pollution sinks. India, the world's most populous country, is also home to 13 of the top 20 most polluted cities, with inconsistent environmental standards and a massive economic growth fuelled by coal contributing to its low air quality.

Air pollution's impact on health

Air pollution is a serious health threat that can trigger asthma attacks, harm lung development in children, and even be deadly. The health risks from air pollution vary depending on age, location, underlying health, and other factors. People with lung diseases such as asthma, chronic bronchitis, emphysema, and chronic obstructive pulmonary disease are particularly vulnerable to the health impacts of air pollution.

Ozone air pollution, also known as ground-level ozone, is a powerful lung irritant. When inhaled, ozone reacts with the delicate lining of the small airways, causing inflammation and damage that can impact multiple body systems. High ozone levels can cause breathing problems such as chest tightness, coughing, and shortness of breath, even in otherwise healthy young adults.

Particle pollution, including carbon, sulphates, nitrate, and toxic heavy metals, can have both acute and long-term health effects. Short-term exposure to fine particles in the air can aggravate lung disease, trigger asthma attacks, and increase the risk of respiratory infections. Prolonged exposure to particle pollution can increase the risk of chronic inflammatory lung disease and other non-communicable chronic diseases affecting the brain, heart, liver, and kidney.

Wood smoke, which contains wood tars, gases, soot, carbon monoxide, dioxins, volatile organic compounds (VOCs), and fine particles, poses serious health risks. Short-term exposure to wood smoke can have similar effects to particle pollution, while long-term exposure increases the chances of developing chronic obstructive lung disease (COPD), chronic bronchitis, cardiovascular disease, or lung cancer.

Certain populations are disproportionately affected by air pollution. Low-income communities and minority populations are often exposed to higher levels of air pollution and are more vulnerable to adverse health impacts. People of color are more likely to live with chronic conditions that make them more susceptible to the health impacts of air pollution. Psychosocial stressors such as poverty, racial/ethnic discrimination, and residency status can also amplify the harmful effects of air pollution.

Air Quality Index (AQI)

The Air Quality Index (AQI) is a tool developed by the US Environmental Protection Agency (EPA) to communicate information about outdoor air quality and health. The AQI includes six colour-coded categories, each corresponding to a range of index values. The higher the AQI value, the greater the level of air pollution and the more serious the health concern.

The six categories are:

• Good: AQI value of 50 or below
• Moderate: AQI value between 51 and 100
• Unhealthy for sensitive groups: AQI value between 101 and 150
• Unhealthy: AQI value between 151 and 200
• Very unhealthy: AQI value between 201 and 300
• Hazardous: AQI value of 301 or above

AQI values at or below 100 are generally considered satisfactory. When AQI values are above 100, air quality is unhealthy for certain sensitive groups of people and then for everyone as AQI values increase.

The AQI is used to report on the five major air pollutants regulated by the Clean Air Act. Each of these pollutants has a national air quality standard set by the EPA to protect public health. The AQI provides information on the health effects of these pollutants and how to avoid adverse health effects. For example, individuals can check the air quality forecast in their community and avoid exercising or working outdoors when unhealthy air is expected.

Ozone pollution

Ground-level ozone, or tropospheric ozone, is distinct from the "good" ozone found in the upper atmosphere (stratospheric ozone). The ozone layer in the upper atmosphere plays a crucial role in shielding us from the sun's harmful ultraviolet radiation. In contrast, ground-level ozone is a harmful air pollutant due to its detrimental effects on human health and the environment.

Climate change exacerbates the conditions for ozone pollution formation and hinders efforts to improve air quality in communities with high ozone levels. Addressing ozone pollution requires collective action from policymakers, industries, and individuals to reduce emissions and enforce regulations that protect public health and the environment.

Particle pollution

There is no safe threshold for breathing in fine particles, and particle pollution is associated with increased mortality from all causes. It is important for individuals to be aware of air quality forecasts and take precautions when particle pollution levels are expected to be unhealthy.

Worst-polluted cities

The World Health Organization (WHO) has documented the world's worst air pollution, with a list of the top 500 cities by PM2.5 annual mean concentration measurement. PM2.5 refers to particles smaller than 2.5 micrometers. The following cities are among the worst-polluted cities in the world:

Byrnihat, India

Byrnihat, located on the Assam-Meghalaya border, has been named the most polluted city in the world, with a PM2.5 concentration of 128.2 µg/m³. This is far beyond the safe limits recommended by the WHO, and breathing the air in Byrnihat is as harmful as smoking nearly three cigarettes a day.

Delhi, India

Delhi has been ranked the world's most polluted capital for six consecutive years, recording an annual PM2.5 level of 108.3 µg/m³. While Delhi's air quality has improved as of March 17, 2025, the PM2.5 level is still 2.67 times higher than the WHO's safe limit.

Mullanpur, India

Mullanpur, a town in Punjab, India, was ranked the fourth most polluted city in the world in 2024, with a PM2.5 concentration of 102.3 µg/m³. As of March 17, 2025, the Air Quality Index (AQI) is 124, which is considered 'Unhealthy'.

Lahore, Pakistan

Lahore, Pakistan's second-largest city, is facing serious air pollution problems. In November 2024, Lahore's AQI shot up to 1,200, four times the hazardous level, making it extremely dangerous to breathe.

Dera Ismail Khan, Pakistan

Dera Ismail Khan, a city in Khyber Pakhtunkhwa, Pakistan, was ranked among the top 10 most polluted cities in the world in 2024, with a PM2.5 level of 93 µg/m³. As of March 17, 2025, the AQI is 96, which is considered 'Poor'.

N'Djamena, Chad

N'Djamena, the capital of Chad, has been facing serious air pollution issues. Chad is often buffeted by dust storms, which contribute to higher PM2.5 concentrations in the country. The use of biomass fuels and traditional stoves for cooking also causes harmful levels of indoor air pollution that can spread outdoors.

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