Commercial replacement of fluorocarbons in mobile air conditioning systems (MACS) with typically HC-290/600a[60/40] and minimal precautions began in Idaho in 1993. R290/600a has low environmental impact but mixed with air is flammable and frequencies for fire, injury and cabin overpressure were predicted. No such accidents are known from 1993 to mid-2003. R290/600a suppliers consumption, representative MACS charges and charge termination frequency were used to predict yearly total MACS numbers. Mid-2002 in Australia there were 0.33 ◊ 10 6 and at end 2002 in the USA 4.7 ◊ 10 6 R290/600a MACS. Integrating over time gave usage for Australia to the end 2002 as 1.09◊10 6 car-years and to mid-2003 for the USA 21.7 ◊ 10 6 car-years. The measured US fire, injury and cabin overpressure frequencies are less than 3.2◊10 7 per car year with high probability. This fire frequency is a hundred times lower than predicted because most R290/600a leaks in MACS use are eectively nonflammable.