Traffic has been the main source of air pollution in Bangkok, which reached serious levels in the 1990s. But efforts to improve air quality by improving fuel quality and enforcing emission standards, among others, had visibly ameliorated the problem by the 2000s. Atmospheric particulate matter levels dropped from 81 micrograms per cubic metre in 1997 to 43 in 2007. However, increasing vehicle numbers and a lack of continued pollution-control efforts threatens a reversal of the past success. In January–February 2018, weather conditions caused bouts of haze to cover the city, with particulate matter under 2.5 micrometres (PM2.5) rising to unhealthy levels for several days on end.