More than two million Chinese youths die each year from health problems related to indoor air pollution, with nearly half of them under five years of age, state media cited a government study as saying.

The study released by the China Centre for Disease Control and Prevention said indoor pollution levels can often be 5-10 times higher than those measured in the nation's notoriously bad outdoor air, the China News Service said.

This indoor pollution causes respiratory and other conditions that kill 2.2 million youths each year, one million of whom are under the age of five, the report said, citing the study released on Sunday.

AFP was not immediately able to obtain a copy of the study.

The study said dangerous indoor pollutants include formaldehyde, benzene, ammonia and radon.

It said formaldehyde posed the biggest threat. It is often found in building materials and new furniture in China and can be slowly released into indoor environments over the course of several years.

It said long-term exposure to such substances can cause a range of health problems including respiratory diseases, mental impairment and cancer, with young children, foetuses in utero and the elderly at most risk.

China's massive economic expansion of the past three decades has made it one of the world's most polluted countries as environmental and health concerns are trampled amid an overriding focus on industrial growth.

Countless cities are smothered in smog while hundreds of millions of citizens lack access to clean drinking water.

A 2007 World Bank report said 750,000 Chinese die prematurely each year due to air and water pollution -- a figure edited out of final versions of the report, reportedly after China warned it could cause social unrest.