Chlorine (Cl2) has a strong warning property: most people can detect Cl2 at approximately 0.002 ppm in air (the official odor threshold per the Agency for Toxic Substances and Disease Registry) and 0.31 ppm in water. However, most data indicate an odor threshold of 0.2-0.4 ppm. In other words, even the most minute leak will be detected by most humans, provided that your process is not allowed to run with known leaks. However, some chlorine rooms and processes are large and complex, and identifying the exact leak point can be challenging, especially when 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?