Investigators with the state Fire Marshal’s Office say two separate fires in York County this morning were caused by men smoking while using oxygen and both men were seriously injured.

David MacDonald, 64, of 18 Boothby Lane in Saco was treated at Maine Medical Center after a fire erupted when he lit a cigarette while in bed at 3:30 a.m.

Five other people living in the duplex were able to escape without injury but the home was destroyed.

A fire at the home of David Morse, 62, of Owl Ridge Road in Limerick started when he began smoking on the living room couch, investigators said. The fire broke out at 5:20 a.m. in a home Morse shares with a relative.

That fire also destroyed the house.

Oxygen is often used as therapy for people who have smoking-related lung disease. But the gas can increase the speed at which fire burns and can create a fireball. Also, oxygen can saturate clothing, increasing the risk of fire injuries.

MacDonald was the most seriously injured and could face an extensive convalescence. Morse was less seriously injured but still being kept for observation, according to the Fire Marshal’s Office.