Over the past fortnight, haze from Indonesian forest fires has covered Singapore. The island state's Pollution Standards Index rose to an all-time high of 401 on June 21 -- anything above 300 is considered hazardous to human health. This comes with a recommendation that residents, particularly children, the elderly and others vulnerable to respiratory disease, "avoid unnecessary outdoor activity". Schools and airports in some Asian countries have been closed. The smoke can be seen in the above satellite image from NASA.