Explain why body substance isolation is generally the preferred method of isolation used by pre-hospital and out-of-hospital providers

Body substance isolation (BSI) is generally the preferred method of isolation because it gives a good amount of protection against infection. 
Infection can be spread via body substance so by isolating it you can prevent it. Patient out of hospital disease mostly is unknown and it's better to do a stricter precaution. In a case of new disease emerge, BSI practice can delay the spreading of disease.