Carbon dioxide is one of the most frequently overlooked of all toxic gases. Even to refer to CO2 as a toxic gas, is a surprise to many safety professionals. Let’s take a closer look.

In the past, the majority of atmospheric monitoring programs have treated CO2 as only a “simple asphyxiant”. An asphyxiant is a substance that can cause unconsciousness or death by suffocation (asphyxiation). Asphyxiants, which have no other health effects, are referred to as “simple” asphyxiants.

As carbon dioxide was not considered to be a toxic hazard, rather than directly measuring the CO2 concentration in a confined space or workplace environment, it was seen as adequate to simply measure the oxygen concentration. But is this assessment really valid?

The effects of CO2

To answer this question, it helps to look at the physical effects of different CO2 concentrations as listed by some Health Authorities. The levels of CO2 in the air and potential health problems are:

Source: Wisconsin Department of Health Services

The Food Safety and Inspection Service of the U.S. Department of Agriculture goes even beyond those concentrations when listing the symptoms for short time exposure:

Source: Carbon Dioxide Health Hazard Information Sheet by fsis.usda.gov

It also states: “The response to CO2 inhalation various greatly even in healthy individuals. The seriousness of the symptoms is dependent on the concentration of CO2 and the length of time a person is exposed. Since CO2 is odorless and does not cause irritation, it is considered to have poor warning properties. Fortunately, conditions from low to moderate exposures are generally reversible when a person is removed from a high CO2 environment.”

Those symptoms are not a result of oxygen deficiency, but a direct effect of CO2 concentration. It’s true, many gases are toxic at much lower concentrations, but if the concentration is high enough, so is CO2. It is also important to remember that people react differently to the same gas concentrations and might show symptoms or suffer damage earlier.

The relation between O2 and CO2

Not even the Wisconsin Department of Health Services makes a clean distinction between the effects of carbon dioxide and a lack of oxygen: “40.000 ppm = This level is immediately harmful due to oxygen deprivation.” However, the correlation is not quite that simple.

Carbon dioxide is a primary by-product of bacterial decomposition. In many confined spaces, there is a direct relationship between low concentrations of oxygen and elevated concentrations of CO2.

In the case of a confined space where CO2 is generated as a by-product of aerobic bacterial action, a concentration of 19.5 % O2 (the hazardous condition threshold for oxygen deficiency in most jurisdictions) would be associated with an equivalent concentration of at least 1.4 % (= 14,000 ppm) CO2. This is substantially higher than the generally accepted workplace exposure limit for CO2 (5,000 ppm calculated as an 8-hour TWA).

The true concentration of CO2 could be substantially higher, if the oxygen deficiency is due to displacement rather than consumption of the oxygen in the confined space. Fresh air contains only 20.9% oxygen by volume. The balance consists mostly of nitrogen, with minor or trace concentrations of a wide variety of other gases including argon, water vapor and carbon dioxide.