Household appliances, such as gas fires, boilers, central heating systems, water heaters, cookers and open fires which use gas, oil, coal and wood may be possible sources of CO gas if the fuel does not burn fully. If household appliances are well serviced and used safely they should produce negligible quantities of CO gas. The older the appliance, the less frequently it is serviced, the higher the chance is that higher levels of CO gas will be emitted. Excess CO gas can build in rooms where people can inhale it. Smoking cigarettes causes a rise in CO blood levels. A switched-on car produces CO gas. A car in a closed garage with the engine running can cause lethal levels of CO gas within ten minutes. Burning charcoal also produces CO gas. Blocked flues and chimneys can stop CO gas from escaping. Fumes from paint removers and cleaning fluids containing methylene chloride (dichloromethane) may cause CO gas poisoning. Methylene chloride turns into carbon monoxide when it is breathed in.