One technique of measurement of the amount of filterable particulate matter suspended in air which has been used in the past depends upon drawing a measured sample of air (usually 1 000 linear feet) through a paper or @MT06878@ @F02384@. A measurement is made of the intensity of light transmitted through the dust spot formed relative to that transmitted through an identical clean @F02384@. The dirtiness of the air is reported in terms of the \(\text{COH}\) unit. This relates to the quantity of particulate material which produces an @O04306@, \[\log _{10}(\frac{I_{0}}{I})\] of 0.01 when measured by light @T06477@ at \(400\ \text{nm}\) and relative to the @T06477@ of an identical dust-free @F02384@ taken as 100%. Thus a @F02384@ which transmitted 50% relative to the clean @F02384@ has an @A00028@ of 0.301 or 30.1 \(\text{COH}\) units. This is not a recommended measure of filterable particulate matter since the size, colour and other properties of the @A00176@ and the air in which it is suspended affect the results.