Excessive air pollution is a growing problem in poor and middle-income cities which causes death on about 3 million people. According to the U.N. health agency more than four out of five city dwellers live in cities that have poor air quality since they do not meet with the guidelines of WHO.

"Ambient air pollution, made of high concentrations of small and fine particulate matter, is the greatest environmental risk to health, causing more than 3 million premature deaths worldwide ever year," WHO said, revealing that 98 percent of poorer countries and 56 percent of high-income countries have rising air pollution.

The report is based on the recent third Global Urban Ambient Air Pollution Database of WHO which examined the outdoor air of 3,000 cities, towns and villages in 103 countries from 2008 to 2013.

Findings reveal that air pollution is increasing at 8 percent "despite improvements in some regions" during said span of time. Because of the quality of air, WHO noted that people are facing a higher risk of heart disease, lung cancer, strokes, and other respiratory diseases. Dust, smoke and soot may penetrate to the lungs and cause these diseases.

"Urban air pollution continues to rise at an alarming rate, wreaking havoc on human health," said Dr. Maria Neira, a WHO director for environment and public health, Fox News reported. "At the same time, awareness is rising and more cities are monitoring their air quality. When air quality improves, global respiratory and cardiovascular-related illnesses decrease."

Zabol city of Iran was ranked the first of the world's worst air pollution. It is followed by Gwalior and Allahabad in India, and Riyadh and Al Jubail in Saudi Arabia. Patna and Raipur from India ranked sixth and seventh. Meanwhile, India's capital, Delhi previously topped the list but is now at rank 11, Wall Street Journal reported.