Why so smoggy? Farmers in Northern Thailand burning the fields to allow for replanting and regrowth. In 2009, the Irawaddy noted that “The traditional rural method of slash-and-burn farming, whereby fields are burned by farmers in the dry season between February and April, so that the ashes fertilize the fields while they lie fallow, is responsible for the greater part of the pollution.” And they’re so bad in the far North that there is talk of evacuation near Mae Sai.