In 1967, in response to widespread public concern aroused by medical reports of asbestos(石棉) related deaths, the National Medical Research Council organized a committee of inquiry to investigate the health threats associated with the use of asbestos in the building industry. After examining evidences provided by medical researchers and building workers and management, the Council published a report which included advice for dealing with asbestos. The report confirming the findings of similar research in the United States and Canada. Exposure to relatively small quantities of asbestosis fibers, they concluded, was directly responsible for" the development of cancers, asbestosis(石棉沉滞症) and related "diseases. Taking into account evidence provided by economists and building industry management, however, the report assumed that despite the availability of other materials, asbestos would continue to play a major role in the British building industry for many years to come because of its availability and low cost. As a result, the council gave a series of recommendations which were intended to reduce the risks to those who might be exposed to asbestos in working environments. They recommended that, where possible, asbestos-free materials should be employed. In cases where asbestos was employed, it was recommended that it should be used in such a way that loose fibers were less likely to enter the air. The report recommended that special care should be taken during working in environments which contain asbestos. Workers should wear protective equipment and take special care to remove dust from the environment and clothing with the use of vacuum cleaner. The report identified five factors which determine the level of risk involved. The state and type of asbestos is critical to determining the risk factors. In addition, dust formation was found to be limited where the asbestos was used when wet rather than dry. The choice of tools was also found to affect the quantities of asbestos particles that enter the air. Machine tools produce greater quantifies of dust than hand tools and, where possible, the use of the latter was recommended. A critical factor that takes place in risk reduction is the adequate ventilation of the working environment. When work takes place in an enclosed space, more asbestos particles circulate and it was therefore recommended that natural or machine ventilation should be used. By closely following this advice, it was claimed that exposure could be reduced to a reasonably practical minimum. It can be inferred from the passage that the real er comes from______