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.
Carbon dioxide is the fourth most common gas present in the earth’s atmosphere, with an average ambient concentration (in fresh air) of about 350 ppm. Carbon dioxide is one of the most common byproducts of living organisms. With every exhaled breath we produce and release CO2 into the atmosphere (with an average concentration in exhaled breath of about 3.8%). According to one USDA study, an average person produces about 450 liters (900 grams) of CO2 per day.
Liquid and solid carbon dioxide (dry ice) are widely used as refrigerants, especially in the food industry. Carbon dioxide is also used in many industrial and chemical industry processes. Carbon dioxide is particularly associated with the beer and wine making industries, where it is produced as a byproduct of fermentation. Carbon dioxide is also widely used in the oil industry, where it is commonly injected into oil wells to aid the decrease the viscosity and aid in the extraction of oil from mature fields. It is also one of the most common atmospheric hazards encountered in confined spaces.
The infrared absorbance spectrum of carbon dioxide shows a strong peak at 4.3 µm
Carbon dioxide is a primary byproduct of bacterial decomposition. As with people, “aerobic” or oxygen using bacteria produce carbon dioxide as a primary metabolic byproduct. 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 byproduct 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 workplace exposure limit for CO2 in most jurisdictions (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. Because oxygen represents only about one-fifth of the total volume of fresh air, every 5% of a displacing gas that is introduced into a confined space reduces the oxygen concentration by only 1%. As an example, consider an oxygen deficiency due to the introduction of dry ice into an enclosed space. In this case a reading of 19.5% O2 would not be indicative of 1.4% CO2, it would be indicative of 5 X 1.4% = 7.0% (= 70,000 ppm) CO2.
The bottom line is that if you wait until the oxygen deficiency alarm is activated, and the deficiency is due to the presence of CO2, you will have substantially exceeded the toxic exposure limit before leaving the affected area.
In spite of these considerations, 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. Because CO2 was not considered to be a toxic hazard, rather than directly measuring the CO2 in the confined space or workplace environment, it was seen as adequate to simply measure the oxygen concentration. This attitude is changing as it becomes more feasible to directly measure CO2 concentration by means of compact, portable multi-sensor gas detectors equipped with miniaturized infrared sensors for the direct measurement of this gas.
Carbon dioxide is a toxic contaminant with strictly defined workplace exposure limits
Carbon dioxide is listed as a toxic contaminant with strictly defined occupational exposure limits in almost every jurisdiction. The most widely recognized exposure limits for CO2 reference an 8-hour TWA of 5,000 ppm, with a 15-minute STEL of either 15,000 ppm or 30,000 ppm. The following table lists several of the most commonly cited workplace exposure limits:
Carbon dioxide is heavier than air, with a density of 1.5 times that of fresh air. While present as a natural component in fresh air, at higher concentrations exposure symptoms include headaches, dizziness, shortness of breath, nausea, rapid or irregular pulse and depression of the central nervous system. Besides displacing the oxygen in fresh air, high concentrations of CO2 may exacerbate or worsen the symptoms related to oxygen deficiency, and interfere with successful resuscitation.
Concentrations of 40,000 ppm or higher should be regarded as immediately dangerous to life and health. Exposure to very high concentrations (e.g. exposure to 6% volume CO2 for several minutes or 30% volume CO2 for 20-30 seconds), has been linked to permanent heart damage, as evidenced by altered electrocardiograms. Concentrations greater than 10% are capable of causing loss of consciousness within 15 minutes or less.
How NDIR (non-dispersive infrared) CO2 sensors detect gas
The most widely used technique for real-time measurement of carbon dioxide is by means of non-dispersive infrared (NDIR) sensors that measure CO2 as a function of the absorbance of infrared light at a specific wavelength.
Miniaturized NDIR sensors directly measure CO2 by means of absorbance of infrared light
Molecules can be conceptualized as balls (atoms) held together by flexible springs (bonds) that can vibrate (stretch, bend or rotate) in three dimensions. Each molecule has certain fixed modes in which this vibratory motion can occur. Vibrational modes are dictated by the nature of the specific bonds that hold the molecule together. The larger the molecule, the greater the number of modes of movement. Each mode represents vibrational motion at a specific frequency. The modes are always the same for a specific molecule. Chemical bonds absorb infra-red radiation. The bond continues to vibrate at the same frequency but with greater amplitude after the transfer of energy. For infrared energy to be absorbed (that is, for vibrational energy to be transferred to the molecule), the frequency must match the frequency of the mode of vibration.
Specific molecules absorb infrared radiation at precise wavelengths. When infrared radiation passes through a sensing chamber containing a specific contaminant, only those wavelengths that match one of the vibration modes are absorbed. The rest of the light is transmitted through the chamber without hindrance. The presence of a particular chemical group within a molecule thus gives rise to characteristic absorption bands. Since most chemical compounds absorb at a number of different frequencies, IR absorbance can provide a "fingerprint" for use in identification of unknown contaminants. Alternatively, for some molecules it may be possible to find an absorbance peak at a specific wavelength that is not shared by other molecules likely to be present. In this case absorbance at that particular wavelength can be used to provide substance-specific measurement for a specific molecule. Carbon dioxide has such an absorbance peak at a wavelength of 4.3 microns (µm). Absorbance of infrared light at this wavelength is directly proportional to the concentration of CO2 present in the sensing chamber of the sensor.
Miniaturized NDIR CO2 sensors include an infrared light source (typically a tungsten filament lamp) capable of emitting light in the desired wavelengths. Optical filters are used to limit the light transmitted through the sensing chamber to a narrow range of wavelengths. Pyroelectric detectors capable of measuring absorbance at the specific wavelength of interest are used to provide the measurement signal. Most NDIR CO2 sensors are dual detector systems that provide both a reference and an active signal. The amount of light that reaches the active detector is proportional to the concentration of CO2 present in the sensing chamber. The greater the concentration of CO2, the greater the reduction in the amount of light that reaches the detector when compared to the reference signal.
In the past, infrared based instruments have tended to bulky, expensive and required a high level of operator expertise to obtain accurate readings. A new generation of miniaturized NDIR sensors has permitted the development of infrared based instruments for an ever widening variety of contaminants including carbon dioxide, Freons?, ammonia, and methane, as well as generalized hydrocarbon combustible gas detection.
The regulations are already changing. Recent fatalities in the wine industry in California have heightened concerns, and increased the obligation for direct CO2 measurement during workplace procedures that may expose workers to this contaminant. In Germany and Austria regulations already require direct measurement of CO2 during most confined space entry procedures. It is clear that with the increased availability, and increasingly affordable cost of miniaturized NDIR CO2 sensors, more and more atmospheric monitoring programs will include the direct measurement of this dangerous atmospheric contaminant.
About the author:
Robert Henderson is Vice President, Business Development for BW Technologies. Mr. Henderson has been a member of the American Industrial Hygiene Association since 1992. He is the 2006 Chairman of the AIHA Gas and Vapor Detection Systems Technical Committee as well as a current member and past chair of the AIHA Confined Spaces Committee. He is also a past chair of the Instrument Products Group of the Industrial Safety Equipment Association.
Robert E. Henderson
Vice President, Business Development
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