The dry forest soil leads to lower water intake for trees to extract when the rains arrive. In February 2016, Director-General Chatchai Promlert of the Disaster Prevention and Mitigation Department, said that the haze affecting northern Thailand has reached levels that can be considered harmful to health. He said that the Pollution Control Department had reported that the levels of particulates measuring less than 10 micrometres—known as PM10—had crossed the prescribed safe threshold of 120 in four out of nine provinces where monitoring was conducted. The level of PM10 in the nine regions—Chiang Rai, Chiang Mai, Lampang, Lamphun, Mae Hong Son, Nan, Phrae, Phayao and Tak—was measured at between 68 and 160. The haze level was considered unhealthy in Chiang Mai, Lampang, Lamphun, and Phrae Provinces. During the burning season 2016 (February–April), air pollution has shown no improvement despite the government's purported efforts to ameliorate the burning. The Mae Sai District of Chiang Rai Province recorded a record 410 μg/m3 of harmful air particles in the early morning of 25 March 2016. From January–July 2016 the five Thai cities with the highest annual average concentrations of PM2.5 were Chiang Mai, Lampang (Mae Moh), Khon Kaen, Bangkok and Ratchaburi. Seven out of the eleven cities measured (63.6%) did not reach the National Ambient Air Quality Standard annual limit of 25 μg/m3 for PM2.5 and all 11 cities measured did not reach the World Health Organization (WHO) guideline annual limit of 25 μg/m3. Thailand's national air quality standards are weak when compared to WHO recommendations. In the first six months of 2017, Greenpeace Thailand monitored PM2.5 in 14 provinces, as they have done since 2015, and found that every station recorded levels higher than the WHO recommendation of less than 10 milligrams per cubic meter of air. PM2.5 refers to airborne particulates smaller than 2.5 microns, particles so small that they can be inhaled into the blood system and cause cancer and heart disease. Chiang Mai, Tak, Khon Kaen, Bangkok, and Saraburi were among the worst cities with the highest PM2.5 levels in 2017. In February 2018 and 2019, Bangkok suffered under a haze of smog and ultra-fine dust. The Pollution Control Department issued warnings that particulate levels had soared to 94 micrograms per cubic metre of air in some areas, almost double the safe limit of 50 mcg.