The average air quality in 45 major cities - nearly four out of every 10 major cities in the country - was rated as "poor" in the first half of this year, according to the Ministry of Environmental Protection.

Urumqi, capital of Northwest China's Xinjiang Uygur autonomous region, recorded the highest level of pollution.

Air quality in the other cities with poor air quality, mostly provincial capitals or industrial hubs, was rated as 3 on the country's 4-grade pollution scale, meaning the air is slightly polluted.

Grade 1 is the best level.

Other cities with average air quality of Grade 3 include Nanjing in Jiangsu province, Harbin in Heilongjiang province, Chengdu in Sichuan province, Xi'an in Shaanxi province and Chongqing municipality.

Air quality in Beijing was also rated as poor due to the large amount of vehicle emissions, as there are more than 4.9 million cars in the capital.

Although emissions of sulfur dioxide and measured particulate matter dropped compared to the same period last year, Beijing discharged 1.9 percent more nitrogen dioxide in the first six months this year.

Nitrogen dioxide can irritate the lungs and lower people's resistance to respiratory infections like influenza. Continued exposure to higher concentrations can also cause increased incidence of acute respiratory illness among children.

Experts said vehicle emissions and coal consumption are to blame for the worsening nitrogen oxide emissions in Beijing, despite its stringent limit on car purchases and efforts to reduce car use.

Controlling nitrogen emissions from vehicles remains a challenge worldwide.

Currently, 113 major cities in China publish daily reports on the Air Pollution Index. The air quality is rated as polluted when the index is higher than 100. Cities with air quality of Grade 3 have an average Air Pollution Index of 100 to 150.

However, experts said the air quality in major Chinese cities may be even worse if the evaluation included measures of even smaller particulate matter - tiny airborne pollutants that cause haze and are more damaging to people's respiratory system.

The particulate matter normally measured is smaller than 10 microns, and is known as PM 10, while matter smaller than 2.5 microns is called PM 2.5.

Officials from the Ministry of Environmental Protection said in 2009 that PM 2.5 is likely to be included in a set of new standards for air quality appraisal. But in the two years since this announcement, environmentalists said it is unlikely to happen in the short run, citing strong opposition from coal-fired plants and other industries relying on coal for energy.

According to the ministry, 26 cities will pilot new air quality monitoring and appraisal systems for air quality, which include total suspended particulates and the concentration of lead in the atmosphere.

A separate report on water quality in the country's major rivers and lakes, also published by the ministry, said the Haihe River in North China is the dirtiest, with large sections of the river being heavily polluted.

Heavy metal pollution is the most severe in Haihe and the rivers in Southwest China, according to the report.

Dianchi Lake in Yunnan province, with its water rated as dangerous for human contact, is the worst polluted lake, suffering chronically from excessive nutrients that can cause algae bloom.