The committee understood that the main cause of health problems was dust particles less than 5 microns in diameter .
The committee thought silica dust was the main threat , rather than coal dust , and therefore argued that protection was only needed for men exposed to silica dust .
The committee understood that the main cause of health problems was dust particles less than 5 microns in diameter , but thought silica dust was the main threat , rather than coal dust , and therefore argued that protection was only needed for men exposed to silica dust .