For over a century CO2 has been recognised as a workplace hazard at high concentrations. However, CO2 is naturally present in the air we breathe at a concentration of about 0.037% and is not harmful to health at low concentrations. At room temperature and atmospheric pressure CO2 is a colourless and odourless gas and, because of this, people are unable to see it or smell it at elevated concentrations. CO2 is not flammable and will not support combustion. As the concentration CO2 in air rises it can cause headaches, dizziness, confusion and loss of consciousness. Since CO2 is heavier than air, fatalities from asphyxiation have occurred when, at high concentrations, it has entered confined spaces such as tanks, sumps or cellars and displaced Oxygen. It is also possible for CO2 to accumulate in trenches or depressions outside following leaks and this is more likely to occur following a pressurised release where the released CO2 is colder than the surrounding air.
In GB, CO2 is classed as a ‘substance hazardous to health’ under the Control of Substances Hazardous to Health Regulations 2002 (COSHH). The HSE publication ‘EH40/2005 Workplace exposure limits’ provides workplace exposure limits (WELs) for CO2. WELs are limits to airborne concentrations of hazardous substances in the workplace and are set in order to help protect the health of workers. Workplace exposure is calculated by taking an average over a specified period of time. The WELs for CO2 are: