Old prayer flags are usually burnt when discarded under the belief that the winds will carry the prayers to the gods.

Traditionally, prayer flags were made of natural fibres like cotton and silk but the current market is saturated with versions constructed of polyester and other synthetic materials, which take decades to decompose and emit toxic gases when burnt.

"The prayers might be answered but it is also causing pollution," said Ang Dolma Sherpa, founder of Utpala Crafts, which made the biodegradable prayer flags now adorning Boudhanath stupa.

Sherpa uses cotton and prints prayers and symbols on her flags with water-based paint. The ropes are made of natural fibres instead of nylon. "I am giving an alternative. I hope that people will use them wisely and turn them into compost," she said.

Prayer flags and khadas -- Buddhist scarves used as a greeting or offering -- are also popular fixtures in mountaineering, with climbers carrying them for good fortune and offering them at the summit.

Mountain guide Dawa Yangzum Sherpa took biodegradable flags on her recent expedition to the 5,630-metre (18,471-foot) Yalung Ri peak in eastern Nepal.

"It is very important for them to be biodegradable," she said. "These prayers flags and khadas have an unseen impact."