Emission particulates from combustion sources consist of compounds such as sulfates, nitrates, and unburned compounds. Particulate matter emissions are classified into two categories, PM and PM10, which refer to particulates 10 mm or more and less than 10 mm in diameter, respectively. All particulates pose health problems, but small particulates can be inhaled and can cause more damage to humans than larger ones. PM levels from natural gas are lower than those from oils and coals. High ash in fuel oils and coals can also increase the PM. In utility boilers, electrostatic precipitators, scrubbers, or bag houses are used to remove particulates. These systems increase the installation cost of the plant and may not be justified in small plants. Switching to low ash, low sulfur fuels also helps reduce PM but adds to the cost of fuel.