Title: New Delhi
Content:  In Mercer's 2015 annual quality-of-living survey, New Delhi ranks at number 154 out of 230 cities due to bad air quality and pollution. The World Health Organization ranked New Delhi as the world's worst polluted city in 2014 among about 1,600 cities the organisation tracked around the world. In 2016, United States Environmental Protection Agency listed New Delhi as the most polluted city on Earth and IQAir listed New Delhi as the world's most polluted capital city for the second straight year in year 2019. In an attempt to lessen air pollution in New Delhi, which gets worse during the winter, 
Title: Delhi
Content:  According to the World Health Organization (WHO) Delhi was the most polluted city in the world in 2014. In 2016 WHO downgraded Delhi to eleventh-worst in the urban air quality database. According to one estimate, air pollution causes the death of about 10,500 people in Delhi every year. Air quality index of Delhi is generally moderate (101–200) level between January to September, and then it drastically deteriorates to Very Poor (301–400), Severe (401–500) or Hazardous (500+) levels in three months between October to December, due to various factors including stubble burning, fire crackers burning during Diwali and cold weather. During 2013–14, peak levels of 
Title: Health in Nigeria
Content:  the most unsafe globally (ranked 4th) and four of its major cities – Onitsha, Aba, Kaduna, and Umuahia are among the worst polluted cities in the world in term of particulate matter of size 10micrometer and below (PM10) and the most recent report by WHO indicate that the country’s annual mean concentration PM2.5 is 72ug/m3 far exceeding the recommended maximum of 10ug/m3. l Data from the institutes of Health Metrics and Evaluation on Global disease burden (GBD) was used to ascertain the cause of death and DALYs in Nigeria from 2007 – 2017 and published literature where reviewed. According to World Health data 
Title: Ahvaz
Content:  In 2011, the World Health Organization ranked Ahwaz as the world's most air-polluted city. The reason Ahvaz is so polluted is because of its oil industry. The pollution can be very dangerous, causing different types of diseases, and can harmful to plants.
Title: Global Alliance on Health and Pollution
Content:  The work of the Commission was also covered extensively through special partnerships with high-profile media organizations. In addition, GAHP updates findings from The Lancet Commission on Pollution and Health, and provides a ranking of pollution deaths on a global, regional and country level with Pollution and Health Metrics: Global, Regional and Country Analysis reports. Pollution remains the world’s largest environmental threat to human health, responsible in 2017 for 15% of all deaths globally, and 275 million Disability-Adjusted Life Years. The 2019 report, which uses the most recent Global Burden of Disease data from the Institute of Health Metrics Evaluation, underscores the extent and severity of harm caused by air, water, and occupational pollution.
Title: Environmental issues in Delhi
Content:  and December, due to various factors including stubble burning, fire crackers burning during Diwali and cold weather. In May 2014 the World Health Organization announced New Delhi as the most polluted city in the world. Overcrowding and the ensuing overuse of scarce resources such as water put heavy pressure on the environment. The city suffers from air pollution caused by road dust and industry, with comparatively smaller contributions from unclean engines in transportation, especially diesel-powered city buses and trucks, and 2-wheelers and 3-wheelers with two-stroke engines. Another known cause of pollution is slow moving traffic due to frequent pedestrian crossings.
Title: Air pollution in India
Content:  it essentially tells you the level of pollution in the air in a given city on a given day. The AQI of Delhi was placed under the "severe-plus category" when it touched 574, by the System of Air Quality and Weather Forecasting And Research. In May 2014 the World Health Organization announced New Delhi as the most polluted city in the world. In November 2016, the Great smog of Delhi was an environmental event which saw New Delhi and adjoining areas in a dense blanket of smog, which was the worst in 17 years. India's Central Pollution Control Board now routinely monitors four air pollutants namely sulphur dioxide 
Title: Pollution
Content: Agbogbloshie, Ghana ; Chernobyl, Ukraine ; Citarum River, Indonesia ; Dzershinsk, Russia ; Hazaribagh, Bangladesh ; Kabwe, Zambia ; Kalimantan, Indonesia ; Matanza Riachuelo, Argentina ; Niger River Delta, Nigeria ; Norilsk, Russia Pure Earth issues an annual list of some of the world's worst polluted places. Below is the list for 2007:
Title: List of most-polluted cities by particulate matter concentration
Content:  This list contains the top 500 cities by PM2.5 annual mean concentration measurement as documented by the World Health Organization covering the period from 2008 to 2017. The 2018 version of the WHO database contains results of ambient (outdoor) air pollution monitoring from almost 2700 towns and cities in 91 countries. Air quality in the database is represented by the annual mean concentration of particulate matter (PM10 and PM2.5, i.e. particles smaller than 10 or 2.5 micrometers, respectively).
Title: Pure Earth
Content:  For over a decade, Pure Earth's World’s Worst Pollution Problems reports identified and drew attention to the worst, and most dangerously polluted places on the planet, while documenting and quantifying the startling health and environmental impacts of this neglected problem. The series of reports succeeded in raising global awareness about the extent and impacts of toxic pollution in low- and middle-income countries. All reports are archived at http://worstpolluted.org

Question: most polluted city in the world according to world health organization?
Answer: Zabol.