Turbidity is a measure of the relative clarity of water. Particles that cause turbidity include sediment, clay, silt, inorganic and organic matter, algae, dissolved organic compounds, and plankton and other microscopic organisms. Generally speaking, the cloudier the water, the more turbidity is present. Although some turbidity (< 2 NTU) is considered only a secondary aesthetic concern, higher turbidity levels are often associated with disease-causing microorganisms (such as viruses, parasites and some bacteria). These organisms may cause symptoms such as nausea, cramps, diarrhea, and headaches.