公 (public) + 害 (harm) — literally 'public harm', i.e. damage that affects everyone. Compare 公益 (public benefit, the opposite) and 公敌 (public enemy).
usage
Common in environmental and social-issue writing. 八大公害事件 ('the Eight Major Pollution Incidents') is a stock phrase in Chinese textbooks listing 20th-century industrial disasters such as Minamata disease and the Donora smog.
