would be absorbed by the same mass of carbon dioxide over a period of time. This is generally measured over a 100-year period. ODP measures the relative impact of a refrigerant to the ozone layer, standardized to R-11, which has a value of 1. GWP and ODP vary greatly among the different refrigerants. CFCs are generally the highest impact, with a high GWP and ODP. HCFCs have similar GWP values and medium ODP values. HFCs again have similar GWP values but a zero ODP value. Natural refrigerants have low to zero GWP values and zero ODP values. Natural refrigerants are therefore gaining increased interest to replace HFCs and offer a more sustainable option for refrigeration. Refrigerants Hydrocarbons as refrigerants Pure hydrogen compounds see moderate use in refrigeration. Hydrocarbons are a viable option as refrigerants because, besides providing cooling properties, they are also plentiful and energy efficient. They are rated to be up to 50% more energy efficient than synthetic refrigerants. Hydrocarbons are also environmentally friendly, as they exist in nature and rank low on the global warming potential (GWP) scale. Historically, hydrocarbons have mainly seen use as a refrigerant for industrial chilling and refrigeration, but with the current shift towards natural refrigerants they are starting to see an increase in use in other areas of refrigeration. They are the favored refrigerant of many European countries. Hydrocarbons used as refrigerants include: Methane (CH4) [R-50] Ethane (CH3CH3) [R-170] Propane (CH3CH2CH3) [R-290] Ethylene (CH2CH2) [R-1150] n-butane (CH3CH2CH2CH3) [R-600] Isobutane (CH(CH3)3) [R-600a] Propylene (CH3CHCH2) [R-1270] Pentane (CH3CH2CH2CH2CH3) [R-601] Isopentane (CH(CH3)2CH2CH3) [R-601a] Cyclopentane ((CH2)5) Flammability The main detriment of using hydrocarbons as refrigerants is that they are extremely flammable at higher pressures. In the past, this risk was mitigated by turning hydrocarbons into CFCs, HCFCs, and HFCs, but with the increasing avoidance of such substances, the problem of flammability must be addressed. Refrigeration systems work by pressurizing the refrigerant to a point where it begins to display refrigerant properties, but with the risk of pressurizing hydrocarbons there is a higher level of caution needed for the internal pressure. In order for hydrocarbons to combust, there must first be a release of hydrocarbons which mix with the correct proportion of air, and then an ignition source must be present. The range of flammability for hydrocarbons lie between 1 and 10%, and an ignition source must have an energy greater than 0.25 J or a temperature greater