Sentence:  Sarin is colourless, odourless and tasteless. It causes a range of symptoms, from respiratory failure, eye irritation, and blurred vision to constricted pupils and drooling. People <WSD>foam at the mouth</WSD> when forced to breathe through massive secretions of fluid in their lungs. The expelled foam can be tinged pink with blood. 
Meaning of the word marked with <WSD> tags:  To spew saliva as foam.