Airborne pollutants have been increasing in Mexico City, making the Mexican capital the world's fourth most polluted city, an NGO warned Tuesday.

Particles have been above safe levels since 2004, but in 2008 and 2009, they exceeded the annual average, said Gustavo Alanis, director of the Mexican Environmental Law Center (Cemda).

The director blamed the deteriorating air quality on low temperatures, traffic chaos, high sulphur fuel use and obsolete environmental standards for emissions control.

The use of high sulphur fuel has led to a surge in the concentration of the PM2.5 particles, one of the pollutants most harmful to health, Alanis said.

Mexican standards for assessing air quality in the capital city are less rigid than those recommended by the World Health Organization, giving local habitants the wrong message, Alanis added.

According to air quality data in 20 cities released by the Mexican National Institute of Ecology, toxic air pollutants caused 38,000 deaths from lung cancer, cardiopulmonary diseases and respiratory infections between 2000 and 2009.

Cemda is a non-governmental organization that promotes the right to a healthy environment and environmental protection in Mexico.