Why do some Shvetambara monks cover their mouths with a mukhavastrika?

Shvetambara monks use a mukhavastrika—a piece of cloth held or worn over the mouth—primarily to protect against the accidental ingestion of small insects.12

This practice is rooted in the fundamental Jain ethical principle of ahimsa, or nonviolence toward all living things. For ascetics observing the great vows (mahavratas), ahimsa requires the utmost vigilance to avoid causing injury to any life-form, including insects and microbes, whether knowingly or unknowingly. By covering their mouths, monks prevent these tiny creatures from being inhaled or swallowed, thereby upholding their vow of nonviolence and avoiding the accumulation of karma that would otherwise hinder their spiritual progress. For certain groups, such as the Sthanakavasis and Terapanthis, the use of the mukhavastrika is mandatory at all times.21