Try zero circumstances

There is no justification for any city to ever authorize its police to deploy internationally banned chemical weapons against its citizens.  None.  You can play around with the language like this all day but you're never going to arrive at a just solution until your law says just don't do that. 

The ordinance is designed to prevent the police from using chemical irritants in all but the “absolute most dire circumstances,” according to Banks. However, the current version of the proposal allows police to use tear gas and other “riot control” chemicals to prevent crimes of violence.

Williams said that in response to concerns that the legislation might give too much leeway, he hopes to amend the ordinance so that the only exception is in situations involving an “imminent” threat of loss of life or bodily injury. He also intends to amend the ordinance at a full City Council vote to require police to issue a warning before using gas.

Writing those bullshit qualifiers into the ordinance guarantees that 1) the cops will definitely use tear gas on more people and 2) Williams and Banks will yell at anyone who  complains that they've fixed the problem already.