COVID-19 should not be the world's last public health disaster, so there is an urgent need to learn from COVID-19 to prepare better for the next public health disaster. This study aims to understand the factors that make people wear a face mask at the beginning of an outbreak of public health disaster. Semi-structured interviews were conducted during April 2020 in China, one month after the COVID-19 was declared a pandemic. The respondents were members of the public living in China, covering two age groups: young adults and older adults. They were recruited using a convenient sample and snowball sampling strategy. The results were analysed using content analysis. Seventeen subjects were recruited, among which nine were young adults (average age = 26.4; SD = 10.5), and eight were older adults (average age = 60.4; SD = 12.1). This study found that environmental factors, personal factors, factors concerning wearing masks, specific circumstances, and development of the pandemic were the common factors considered by both young adults and older adults. This study should help the authority formulate prevention policies better to reduce the risk of an outbreak if there is a new virus outbreak in the future, unfortunately.