The first US standard for controlling exposure to asbestos dust was recommended by Dreesen, of the US Public Health Service, in 1938, following a study of 541 employees of four asbestos textile plants in which massive exposures had occurred. A tentative limit of asbestos dust recommended for the textile industry was 5 million particles per cubic foot of asbestos dust as determined by the midget impinger technique and area samplers. The use of asbestos is essential for protection of the public against fire and other risks, and its value must therefore be weighed against the degrees of risk, which arise from its use and the practical difficulties of control. There was much concern in the 1980s and 1990s about the risk to school children exposed to small amounts of asbestos from ceiling tiles and other areas of asbestos found in US schools.