What should you do after coming in contact with tear gas?

During weeks of often violent protests in Hong Kong, police have fired thousands of rounds of tear gas to disperse anti-government protesters. 

Tear gas is essentially a “chemical weapon”, irritates mucous membranes to cause crying, sneezing, coughing and breathing difficulties.

But how might tear gas might affect bystanders as well as protesters in a densely populated city like Hong Kong?  

Doctor David Hui, the Stanley Ho Professor of Respiratory Medicine at the Chinese University of Hong Kong, has some advice about what people can do before – and after – coming in contact with tear gas.