Gas leaks from gas dosing systems are actually very rare. The kind of “deadly chlorine gas kills five” headline in the media almost always refers to spillages of sodium hypochlorite, which will immediately let off gas creating a hazardous situation.
Notwithstanding this, operators always take care when dealing with chlorine gas. Like any compressed gas, it has to be treated with respect.
The human nose can detect chlorine in air at about 3 parts per million (ppm). This is not a lot of chlorine, given that chlorine expands in air from its compressed state. In fact vaporized chlorine gas occupies 460 times the volume in air that it does as a liquid. Chlorine will vaporize instantly when introduced to the atmosphere. The boiling point of chlorine is -28°F!
It is important to detect any release of chlorine as soon as possible using gas detectors. It isn’t possible to post guards (they wouldn’t like it) and in any case they wouldn’t know until the concentration had reached 3ppm.
The installation of permanent gas detectors for continuous monitoring of chlorine (and, in fact, for sulfur dioxide) is mandated in many countries. They are inexpensive and will let you know about any leak that causes an ambient concentration of greater than 1ppm.