Scientists working at the Purdue university in the US, are making progress in perfecting automobiles and portables air-conditioners that use environment-friendly carbon dioxide (CO2) as a refrigerant instead of the conventional, synthetic global-warming and ozone-depleting chemicals - chlorofluorocarbon (CFC) Co2 was the refrigerant of choice during the early 20th century but was later replaced with manmade CFCs.