Air pollution in Malaysia’s Bornean state of Sarawak has surged into hazardous territory, with the air pollutant index (API) topping 400 in the worst-hit district, as forest fires raging in Indonesia continue to send thick smoke drifting across the border.Serian in Sarawak, closest to the frontier with Indonesia’s West Kalimantan province, recorded the highest reading of 476 as of Monday morning.Kuching, Samarahan and Sri Aman all logged readings above 300 as of 10am, according to Malaysia’s Environment Department.The API scale classifies anything above 300 as hazardous; readings of 201-300 are “very unhealthy”, 101-200 “unhealthy”, 51-100 “moderate” and 0-50 “good”.Firefighters spray water in an attempt to extinguish a wildfire in Indonesia’s South Sumatra province on Friday. Photo: ReutersMalaysia has activated its National Open Burning Action Plan and National Haze Action Plan in response, mobilising government agencies to tackle both the fires and the health risks posed by the smoke.