Carbon monoxide is a colourless, odourless gas that is generally a product of incomplete combustion. It is toxic, with the long-term exposure limit being 35 ppm and the maximum exposure limit being 1200 ppm. Carbon monoxide is also flammable between the limits of 12.5 % and 74.5 % in air. It is practically insoluble in water. Carbon monoxide was one of the first pollutants to be targeted for reduction and is still the main component measured worldwide. Limits for carbon monoxide are mostly below 100 ppm for most types of burner. In general, an electrochemical sensor is used to measure carbon monoxide, although infrared measurement is becoming more common. Nearly all instruments are fitted with a sensor for carbon monoxide of some sort, since the value is generally needed and does give valuable information about the combustion process.