A crew enters a building, the passive CO alarm detector goes into alarm. The crew heads out and fires up a 3-gas meter with a CO sensor. They find 2000 ppm in the building. The estimated time that this crew spent in the structure with no respiratory protection was between 2 and 3 minutes.
The IDLH for CO is listed around 1200 PPM. Near fatal exposure is around 1800-1900 PPM. Several members were in the house and nobody has symptoms of exposure with levels even after being the building with CO levels found to be above the fatal exposure level?
The Simple Answer
The simple answer requires us to consider carbon monoxide (CO) as a hazard-that is something that can do harm. In order for carbon monoxide to do harm we have to be exposed to it, that exposure has to been long enough to cause harm, and we must be unprotected from that harm. In this case the crew was exposed to CO and they were unprotected. Assuming that the concentration was sufficient to do harm and it did not means that the crew was not exposed to it long enough for harm to occur.
In order for a high concentration of CO (or anything else for that matter) to do harm those exposed have to have been exposed long enough for the harm to occur. All other things being equal, the person exposed to a concentration of a longer period of time receives a higher total dose.
Other factors at work as well. There are multiple charts available that explain the health effects of CO exposure. Those charts are reasonably accurate but in most cases they are based on a “standardized population” that does not include the very old, the very young, or those with pre-existing medical conditions. What this means is that the average firefighter in good health can likely be exposed to pretty high levels of CO without ill-effects. How high? For how long? Who knows? That is why our action numbers are so conservative!
35 ppm of CO is the time weighted average TWA for CO. Basically that means that if the average CO exposure over an eight hour workday is 35 ppm or less you are good. It is important to focus on the key word “average”. Because it is an average you can exceed the TWA many times throughout the workday with no problems. However, there is a short term exposure limit (STEL). When you hit the STEL you can work there for 15 minutes but then you can’t be back in that space until the level falls below the STEL and the STEL (15 min average) values are still calculated into the TWA (8 hour average). You can only be exposed to STEL limits 4 times in an 8 hour day. The next value is the ceiling limit. This is a value that once you reach it you have to leave. (all of these values assume that there is no respiratory protection [SCBA] in place.) The final value is the IDLH. The IDLH is a point at which a person exposed to that level of CO would become incapacitated after 30 minutes of exposure.
There are multiple charts available that explain the health effects of CO exposure. Those charts are pretty accurate but in most cases they are based on a “standardized population” and does not include the very old, the very young, or those with pre-existing medical conditions. WHat this means is that the average firefighter in good health can likely be exposed to pretty high levels of CO without ill-effects. How high? Who knows? But we don’t take big chances with it. That is why the CO alarm is set at 35 ppm. We acknowledge as an organization that we can survive 35 ppm with no ill effects but there are so many variables we believe that you must mask up at this point.
Incidents such as the one described here highlight the following critical factors: 1.We use ultra conservative numbers when setting thresholds. Personnel are advised to follow them because we cannot account for all the variables. 2.We have to be careful with the words that we use. A crew that enters a structure and announces that there is an IDLH of CO when their monitor alarms at 35 ppm is painting an inaccurate picture. YES! According to our policy this is an action level and you have to mask up BUT there is not an IDLH. 3.The meters we use have a relatively small sample size. It takes approximately one minute for the meter to check the volume of a softball. When you rush into a place and the meter goes into alarm it more than likely alarmed on the stuff you measured one minute ago. 4.Most of the time we meter with the meter at waist height (so we can see it, right?). Well some things are heavier than air (propane) and some things are lighter than air (natural gas). When you are measuring at waist level and get an LEL hit on propane just understand you are walking through an invisible pool of propane. Likewise, when you get an LEL hit for natural gas at waist level you have to be wonder what is above your head. It is perfectly fine to start at the waist but put the meter where the product is to get a real sense of the hazard you face. 5.The OSHA standard definition of IDLH “means an atmosphere that poses an immediate threat to life, would cause irreversible adverse health effects, or would impair an individual’s ability to escape from a dangerous atmosphere.” emphasis added is minimum standard. We establish, as we should, local best practices and policies that are more stringent. 6.Doses take time. This does not mean that we should linger with no SCBA just because 150 ppm is below the STEL of CO but it does mean that we can make a rational, mission oriented, risk assessment-will 200 ppm kill me before I can get these people out? Probably not.