A dense wave of smog began in the Chinese city of Harbin and the surrounding Heilongjiang, Jilin, and Liaoning Provinces on 20 October 2013. Unseasonably warm temperatures with very little wind across northeastern China coincided with the smoke from local farmers' burning straws and the start-up of Harbin's coal-powered central heating system. Record densities of fine particulates were measured in the city. In Harbin, the levels of PM2.5 (particulate matter) rise to 1,000 micrograms per cubic meter, worse than Beijing's historic highs. Visibility was reduced to below 50m in parts of Harbin, and below 500m in most of the neighbouring Jilin Province. On Fa Yuen Street in Harbin, visibility of less than 5m was reported. The smog reduced after October 23, 2013 and completely broke up on October 28, 2013 by the first local snow and icy rain due to a cold front moving in from Russia. Harbin lies in the north of China where winter temperatures can drop to -40degC, demanding a six-month heating season. Daily particulate levels of more than 40 times the World Health Organization recommended maximum level were reported in parts of Harbin. The smog remained till 23 October, when almost all monitoring stations in Heilongjiang, Jilin,and Liaoning Provincesreported readings above 200mg/m3forPM2.5. PM2.5 is the amount of particulate matter less than 2.5 micrometers in diameter in the air, with the WHO recommending a maximum 24-hour mean   of 25 micrograms per cubic meter (mg/m3). On the morning of 25 October, PM2.5 measurements in Harbin had fallen to an average of 123mg/m3. All highways in the surrounding Heilongjiang Province were closed. In Harbin, all primary and middle schools were closed for three days and authorities stopped flights at the airports. Hospitals reported a 23 percent increase in admissions for breathing problems. However, this smog will have no influence on Harbin Ice Festival events later in December 2013. Air pollution in Chinese cities is of increasing concern to China's leadership. Particulates in the air can affect human health and also have influences on climate and rainfall. Pollution from the burning of coal has reduced life expectancy by 5.5 years in the north of China, as a result of heart and lung diseases. One cause of the heavy smog in the northeastern Chinese cities may be   _  .
the lasting cold weather farmers' burning of forests too much strong wind the start-up of heating system