Heartsaver First Aid Part I Progress Indicator: Question 3 of 22 3.  For the purposes of contest work, no barricade will be breached without ventilating in front of the barricade if: Hydrogen Sulfide (H2S) exceeds ___________, or Carbon Dioxide (CO2) exceeds ___________. Choices: 100 ppm H2S or 1.0% CO2 20 ppm H2S or 4.0% CO2 100 ppm H2S or 4.0% ppm CO2 20 ppm H2S or 1.0% CO2