GENEVA, May 7 (Reuters) - An effort by the World Health Organization to measure pollution in cities around the world has found New Delhi admits to having the dirtiest air, while Beijing's measurements, like its skies, are far from clear.

The study of 1,600 cities found air pollution had worsened since a smaller survey in 2011, especially in poorer countries, putting city-dwellers at higher risk of cancer, stroke and heart disease.

Air pollution killed about 7 million people in 2012, making it the world's single biggest environmental health risk, the WHO, a United Nations agency, said last month.

Thirteen of the dirtiest 20 cities were Indian, with New Delhi, Patna, Gwalior and Raipur in the top four spots. The Indian capital had an annual average of 153 micrograms of small particulates, known as PM2.5, per cubic metre.

Beijing, notorious for the smog that has prompted some Anglophone residents to dub it "Greyjing", was in 77th place with a PM2.5 reading of 56, little over one-third of Delhi's pollution level.

WHO experts said the Chinese data was from 2010, the most recent year made available to them by China. But Beijing's city government began publishing hourly PM2.5 data in January 2012.

A year after it started publishing data, Beijing's air quality hit the "worst on record" according to Greenpeace, with a PM2.5 reading as high as 900 on one occasion.

Beijing's government said last month that PM2.5 concentrations stood at a daily average of 89.5 micrograms per cubic metre in 2013, 156 percent higher than national standards. Such a reading would put Beijing 17th in the WHO database. The WHO says there is no safe level for PM2.5 pollution.

At the cleaner end of the table, 32 cities reported a PM2.5 reading of less than 5. Three-quarters of those were Canadian, including Vancouver, one was Hafnarfjordur in Iceland and the other seven were American.

WHO experts insisted the survey was not intended to name and shame the dirtiest cities, since the cities involved were volunteering the information to try to help themselves clean up.

Maria Neira, WHO Director for Public Health, Environmental and Social Determinants of Health, said the aim was to "challenge" cities and thought the survey would help them to become more open about their dirty air, which is often caused by burning coal, smokestack industries and heavy traffic.

She rejected any suggestion that China might be cheating and said it was becoming much more sophisticated about collecting air pollution data, with a new push to clean up the big cities.

"We are very much discussing with China putting on the table the issue of air pollution. Our director general (Margaret Chan) was recently there and she declared that China was one of the countries with major problems with air pollution. We will continue discussions on that to make sure that relevant measures are in place to reduce air pollution."