Annual dry season fires set to clear agricultural land in Sumatra belched a smoky haze over the region, delaying flights in Sumatra and causing dense pollution in Kuala Lumpur, Malacca and Penang and alerts in Singapore. Such fires are illegal but occur every year because enforcement is lax. Hazy conditions can persist for months if fires are allowed to burn and little rain falls. Conditions in recent days were the worst since the 1997-98 season when wildfires burned out of control for weeks and blanketed the region with smoke for several months. Breathing such air can be unhealthful, especially for people with respiratory problems.