You are presented with the quiz "a breathing mask keeps what from being inhaled and causing health problems?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "abreathing mask is used for keeping harmful particles from entering the respiratory system" and "when inhaled, the particles lodge in the respiratory system, causing or aggravating health problems". 

So, what's the best answer to the question?


 

  harmful particles         