Warning Signs

Safety symbols designed to instruct American citizens how to react if terrorists strike may confuse them, according to an article in the latest issue of Ergonomics in Design. For example, the symbol meaning "Use a whistle if one is available. Shout only as a last resort" was interpreted to mean "Yell when you hear a whistle."

I'm most curious what situation requires one to whistle preferably to yelling, in terms of warning others. Is it somehow less dangerous during a biohazard situation?