Pollution can take many forms including chemicals, noise, heat and light. There are four main types of pollution: noise, air, water, and soil. Noise pollution involves unpleasant machine or animal sounds. Air pollution involves harmful chemicals or particulate matter in the atmosphere. Water pollution is the contamination of lakes, rivers and oceans. Soil pollution occurs from chemicals or alterations to the natural soil environment. Preventive measures include noise barriers, developing quieter vehicles, encouraging less vehicle use, adding factory filters, replacing wet industrial processes with dry ones, and afforestation.