On June 19, 2013, NASA's Terra and Aqua satellites captured striking images of smoke billowing from illegal wildfires on the Indonesian island of Sumatra. The smoke blew east toward southern Malaysia and Singapore, and news media reported that thick clouds of haze had descended on Singapore, pushing pollution levels to record levels.
Singapore's primary measure of pollution, the Pollutant Standards Index (PSI)—a uniform measure of key pollutants similar to the Air Quality Index (AQI) used by the U.S. Environmental Protection Agency—spiked to 371 on the afternoon of June 20, 2013, the highest level ever recorded. The previous record occurred in 1997, when the index hit 226. Health experts consider any level above 300 to be "hazardous" to human health. Levels above 200 are considered "very unhealthy."
Both images above were captured by the Moderate Resolution Imaging Spectroradiometer (MODIS), an instrument that observes the entire surface of Earth's every 1 to 2 days. The top image shows smoke blowing east at 3:30 Universal Time (11:30 a.m. local time); the lower image shows the same area on the same afternoon at 6:30 UTC (2:30 p.m. local time).
Though local laws prohibit it, farmers in Sumatra often burn forests during the dry season to prepare soil for new crops. The BBC reported that Singapore's Prime Minister Lee Hsien Loong warned that the haze could "easily last for several weeks and quite possibly longer until the dry season ends in Sumatra."