The reason often varies depending on the Asian country and with varying level of social acceptance. It is not limited to illness (contraction or spreading).

For example, in Taiwan, females will wear the mask to block the sun in an attempt to prevent freckles.

They are also often used to combat pollution. Many choose to only wear while riding scooters, while others any time outdoors.

In some countries masks have become commonplace such that decorative fashionable masks are sold at convenience stores.