The Portland Police Bureau began using tear gas on Black Lives Matters protesters almost as soon as they first assembled in late May. Mayor Ted Wheeler acknowledged that the city has used “CS” tear gas. The commonly used formulation contains 2-chlorobenzalmalononitrile, a compound that was designed to induce immediate pain but can also have long-term effects, including chronic bronchitis. In early September, Wheeler ordered the police to stop using it. Tear gas is banned in war but can be used to disperse crowds of civilians.