Coal is one of the dirtiest forms of energy available.

"There are a number of adverse environmental effects of coal mining and burning, specially in power stations.

These effects include:

Release of carbon dioxide, a greenhouse gas, which causes climate change and global warming according to the IPCC. Coal is the largest contributor to the human-made increase of CO2 in the air.
Generation of hundred of millions of tons of waste products, including fly ash, bottom ash, flue gas desulfurization sludge, that contain mercury, uranium, thorium, arsenic, and other heavy metals
Acid rain from high sulfur coal
Interference with groundwater and water table levels
Contamination of land and waterways and destruction of homes from fly ash spills such as Kingston Fossil Plant coal fly ash slurry spill
Impact of water use on flows of rivers and consequential impact on other land-uses
Dust nuisance
Subsidence above tunnels, sometimes damaging infrastructure
Coal-fired power plants without effective fly ash capture are one of the largest sources of human-caused background radiation exposure
Coal-fired power plants shorten nearly 24,000 lives a year in the United States, including 2,800 from lung cancer.
Coal-fired power plant releases emissions including mercury, selenium, and arsenic which are harmful to human health and the environment."

Source:"Pollution" will have a long-term negative impact on this entity, which subtracts from the entity's value.