Why it is important to use mask in Laboratory?

In some experiment very poisonous gases are evolved. Due to the presence of these poisonous gases student may suffer from headache, vomiting, tears of eyes. Even the students become senseless. These poisonous gases are NO₂ ,  NH₃ , SO₂ , H₂S , CO₂ etc. These gases have great harmful effect. By using the mask students can protect themselves in the laboratory from the effect of these gases. That is why using mask is very important in the laboratory.