We can see some hazardous gases, even at low concentrations, such as Chlorine (Cl2). But some hazardous gases are “clear/invisible”. So, using Anhydrous Ammonia (NH3) as our hazardous gas…
What does 300 ppm of Anhydrous Ammonia (NH3) look like? Note: 300 ppm is the IDLH for NH3
For example, there’s a release of NH3. You can see a white cloud in the release area. But NH3 is a “clear gas.” So, when we can “see” an NH3 release, are we over or under its IDLH?
The simple answer is… WE WILL NEVER SEE 300 ppm of NH3. We will smell it; if we’re NOT wearing a respirator, but we will NOT “see” it.
The reason we can see NH3 is the same way we can see the clouds in the sky. Essentially, NH3 gas in an uncontrolled release from a pressure vessel is COLD. This cold gas condenses the moisture in our atmosphere, thus creating the white cloud. On frigid days where we have less moisture in the air, we say it takes around 4,000 ppm before we begin to “see” the NH3 release. On a humid day with more moisture/humidity, it can be as low as 3,000 ppm. This is 10X’s NH3’s IDLH. Meaning we will never see NH3 at its IDLH.
Hence, it has always been my practice that workers wearing an Air-Purifying Respirator will have a single gas monitor in their breathing zone (e.g., clipped on their collar – NOT on the back of their hardhat). These alarms will have an alarm set point LESS THAN the chemicals IDLH. This is Safety-Critical, as the wearer of the respirator will not be able to rely on smell or sight to recognize a hazardous atmosphere. As we all know, the only respirator that can be worn in an IDLH atmosphere is a Supplied Air Respirator in positive pressure mode (Pressure-Demand). Please note that an SCBA is preferred; however, an air-line respirator may be used when it is used in conjunction with an ESCAPE BOTTLE.