Chains, Ropes, and Slings Test II Source: Questions and answers from the National Safety Council Progress Indicator: Question 6 of 22 6.  According to the 2012 National MNM Mine Rescue Contest rules, above what concentration of NO2 is it not permissible for a mine rescue team to open a barricade without first ventilating in front of the barricade: Choices: 10 ppm 100 ppm 20 ppm 200 ppm None of the above