Chlorine (Cl2) has a great warning property in that most people can smell Cl2 at around 0.002 ppm in the air (the official odor threshold per Agency for Toxic Substances and Disease Registry) and 0.31 ppm in water; although most data says its odor threshold is 0.2 – 0.4 ppm. In other words, even the most minute leak will be detected by most humans – that is if your process is not allowed to run with known leaks! But some chlorine rooms and processes are large and complex and finding the actual leak point can be problematic, especially since time is critical. Remember, Cl2 has an IDLH of 10 ppm and TLV of 1.0 ppm, so looking for a leak point REQUIRES adequate PPE – in my book that entails a Supplied Air Respirator at a minimum! But this article is about the “leak detection” methods – how do we actually find the leak?