In cement kiln operations, brick and structural clay manufacture, energy-from-waste plants and municipal and medical waste incinerators, chlorides present in the fuel source will be converted to HCl during combustion. As most coal contains some amount of chlorine compounds, any coal-fired process will also generate HCl, and the latest proposed regulations have focused on HCI remediation. HCl, corrosive to human tissue, contributes to acid rain and the formation of smog.

Mercury is a toxic, persistent pollutant that accumulates in the food chain. While fossil fuel-fired power plants are the largest remaining source of human-generated mercury emissions in the United States, they contribute only a small amount (about 1%) of total annual mercury emissions worldwide.

Particulate Matter (PM) refers to fine particles and liquid droplets small enough to become airborne. This can include SO₂, SO₃ and fly ash created by industrial combustion processes.

In utility boilers, industrial boilers, cement kilns and other fuel combustors, sulfur in fuel is converted into sulfur oxides (SO₂, SO₃) during combustion. Typically, over 98% of sulfur oxides in the boiler flue gas is SO₂, with the remainder being SO₃. SO₂ and SO₃ contribute to acid rain but there are differences in how these compounds are regulated and treated.

In utility and industrial boilers, cement kilns and other fuel combustors, all sulfur in fuel is converted into sulfur oxides (either SO₂ or SO₃), with approximately 1-2% being SO₃. SO₃ contributes to the formation of condensable or fine particle emissions, known as particulates (PM).