Levels of carbon monoxide emission by aging cars
The hypothesis that a new car will have the lowest amount of carbon monoxide emissions and that the 20-year-old car will have the highest emission levels has been proven to be true.
Among the sources of carbon monoxide are cars left running in enclosed spaces, as well as open fires, water heaters or blocked chimneys. Inhaling high levels of carbon monoxide is harmful and can even cause death. Installation of carbon monoxide detectors in homes is highly recommended to prevent carbon monoxide poisoning.
The experiment may be repeated in different locations, such as an underground car park or a congested road with heavy traffic.
The experiment can also be done to measure the release of other gasses like hydrocarbons, nitrogen oxides or carbon dioxide.
Also, you might want to consider trying to determine if there is a clear correlation between the size of the car's engine and the level of emission of carbon monoxide.
Car exhaust – air pollutants - http://www.nutramed.com/environment/carsepa.htm
Carbon monoxide - http://www.extension.iastate.edu/pages/communications/co/co1.html
Carbon monoxide detector - http://en.wikipedia.org/wiki/Carbon_monoxide_detector
Carbon monoxide gas is extremely poisonous. Use a gas safety mask and ensure the presence of adult supervision when performing measurements.