In the glass liquid helium dewar you will use, there is a vacuum jacket that must be evacuated before use. If you try to transfer liquid helium into the dewar without evacuating the jacket, you will not succeed as the helium will boil off too quickly. If the valve to the jacket is open, you will, however, pump air from the room into the jacket. If you then close the valve and stop transferring liquid helium, the jacket will warm up and the pressure of the air will increase, which will also cause an explosion. This happened several years ago and sent glass clear across the room.
Liquid nitrogen can cause severe frostbite.
Always wear gloves and protective glasses when working with liquid nitrogen. PH156 is accountable to that office for the safe handling and accountability of the sources used in the experiments. During class, Mr. Dean Hudek will instruct you in the use of radioactive material. You naturally receive a radiation dose of 360 millirem per year, or 0.042 millirem per hour, from cosmic rays and other natural sources. The recommended exposure limit for a member of the general public is 100 millirem per year.