Among the dozens of environmental issues the world currently faces, Pollution is likely the worst of them all. According to the World Health Organization, a shocking 92 percent of The World's citizens live in areas that are deemed dangerously polluted by WHO guidelines. The result? Over three million deaths per year from air pollution alone.

Pollution can result from a number of factors, the most common being the burning of coal and fossil fuels, deforestation, and waste produced by factories or power plants. In the following list, I ranked the top 10 most polluted cities in the world by their PM2.5 score, which is calculated by measuring particle matter equal to or under 10 micrometers.

These particles are so microscopic that they are able to enter the lungs and inflict irreversible damage, such as asthma, respiratory issues, or even cancer.

1. Zabol, Iran

Air conditions in Zabol, a south-eastern city in Iran which borders Afghanistan, are as alarming as they are devastating. With a PM2.5 score of 217, Zabol has officially become the world's most polluted city. It is estimated that prolonged exposure to the air in Zabol can cause irreversible damage to one's lungs within hours, which, in the past, has required the city to distribute free mouth masks to its residents and even close down schools and government offices. Some believe that the pollution has been caused by relentless dust storms coming in from western Iran.

2. Gwalior, India

Surpassing Delhi in recent years as the most polluted city in India, Gwalior is the second most polluted our list with a PM2.5 score of 172. This historic city has a population of nearly two million people, many of whom drive cars which contribute to the poor air quality. Fossil fuels, the burning of garbage, the use of biomass for cooking and heating, plus a number of coal plantations in the area all contribute to the city's hazardous air pollution issue.

3. Allahabad, India

Allahabad has a population of over 1.1 million and a PM2.5 score of 170. Engulfed in smog and dust, it is estimated that over 7,000 new Diesel vehicles join Allahabad's roads every month. Other factors that contribute to the city's poor air conditions are mass deforestation, waste from coal plants and unpredictable weather patterns.

4. Riyadh, Saudi Arabia

Riyadh, Saudi Arabia's capital, frequently experiences sandstorms which contains sand that is dangerous to inhale. Additionally, vehicular emissions and waste from nearby factories contribute to the capital's troubling air pollution issue. Riyadh has a popular of over 6.5 million and has a PM2.5 score of 156, making it the worst polluted city in the Middle East.

5. Al Jubail, Saudi Arabia

Known for being the third largest aluminum producer in the world, home to the fourth largest petrochemical company, and the largest industrial city in the Middle East, Al Jubail, located along the Persian Gulf in Saudi Arabia is the fifth most polluted city on my list. With just over 550,000 residents, this gulf city has a PM2.5 score of 152.

Factors that contribute to Al Jubail's air pollution? You guessed it, mass industrial waste emitted from the various factories and power plants.

6. Patna, India

Patna has a PM2.5 score of 149. Considered to be one of the world's oldest cities, Patna is home to over two million people who live surrounded by dust, resulting from numerous construction sites located throughout the city.

7. Raipur, India

Raipur is home to over one million people. The city, known for its sponge iron factories, has a PM2.5 score of 144. Additional factors that contribute to Raipur's poor air quality include overpopulation, vehicular emission, trash and toxic waste from various industrial factories.

8. Bamenda, Cameroon

Nestled in the northwestern region of Cameroon, Bamenda is home to half a million people and has a PM2.5 score of 132.

Bamenda is the only country in Africa on this list and environmentalists believe that deforestation, climate change, dust, and excessive trash accumulated the city's slums are to blame for deteriorating air conditions.

9. Xingtai, China

The most polluted city in China is home to nearly eight million residents, most of whom wear masks anytime they leave the house. Xingtai has a PM2.5 score of 128, mostly a result of its factories who refuse to abide by modern pollution controls. In recent years, there have been fewer than 40 days in the year that met a normal air quality standard.

10. Baoding, China

The second most polluted city in China, Baoding, is just over two hours from the country's capital, Beijing.

The bustling metropolitan's nearly 1.7 million residents are accustomed to wearing masks whenever they leave the house and tourists often mistake the smog for fog. With a PM2.5 score of 126, the thick smog is a result of the numerous coal plantations that provide energy to the city and surrounding towns or villages.

In case you didn't know, April 22 was Earth Day so it's important to our future that we all, no matter where in the world we may be, come together and fight climate change, enforce air pollution policies, and protect our Earth from the damaging effects. Already, humanity possesses the technology and knowledge to properly combat air pollution, it's just a matter of winning the fight against it and all other types of pollution.