Squeezing the nebulizer bulb incorrectly (using 2 fingers) results in an inadequate challenge aerosol concentration.  Proper squeezing of the nebulizer bulb requires the bulb to be completely compressed, then expanded, before the next compression.  Failure to squeeze the nebulizer bulb correctly, can cause poorly fitting respirators to pass the fit test, which can have consequences for the respirator wearer as well as the employer.