There was a tropical storm that generated some very welcome out of season rains in Thailand earlier in the week. This probably is what caused the air quality to improve for a few days. But let's be honest, the air still stayed in the unhealthy range and is now slipping back to horrific levels. When will it end? When the rains become broad and consistent enough which is expected in late May then the air will be able to get to healthy ranges again. That can last till around October when the rains finish then it's back to this all over again.