Taiwan's climate is marine tropical. The Northern part of the island has a rainy season that lasts from January to late March during the southwest monsoon, and also experiences meiyu in May. The entire island succumbs to hot humid weather from June until September, while October to December are arguably the most pleasant times of year. The middle and southern parts of the island do not have an extended monsoon season during the winter months, but can experience several weeks of rain, especially during and after Lunar New Year. Natural hazards such as typhoons and earthquakes are common in the region. With its high population density and many factories, some areas in Taiwan suffer from heavy pollution. Most notable are the southern suburbs of Taipei and the western stretch from Tainan to Lin Yuan, south of Kaohsiung. In the past, Taipei suffered from extensive vehicle and factory air pollution, but with mandatory use of unleaded gasoline and the establishment of the Environmental Protection Agency, the air quality of Taiwan has improved dramatically. Motor scooters, especially older or cheaper two-stroke versions, which are ubiquitous in Taiwan, also contribute disproportionately to air pollution.