CHAPTER 19 SYSTEMS: ENVIRONMENTAL CONTROL AND LIFE SUPPORT SYSTEM—SUPPORTING THE HUMAN ELEMENT OF THE ISS 350 The third emergency scenario is that of a toxic chemical release. In most cases, the cause would involve a chemical released from research equipment and would be cleaned up as described in the previous section. The most serious case is that of ammonia entering the cabin. This would happen if the IFHX (see Chapter 11) fails and the barrier between the outside ammonia and inside water breaks. In this case, poisonous ammonia would get inside, which can kill the crew in a matter of seconds. Once an ammonia leak is detected, the astronauts will don a PBA, remove any clothing since that could be contaminated, close the hatch between the two segments, and transition from the PBA to a respirator with ammonia filters. Since the IFHXs are located in the USOS, there is a good chance the RS will be safe. Shutting down the fans and IMV, which the software performs automatically when the alarm is initiated, will help prevent the ammonia from getting mixed into the RS atmosphere. Once in the RS, the crew will take a reading to determine whether the air is safe. If no ammonia is detected (i.e., ammonia concentration less than 30 parts per million [ppm]), crew members may remove their masks. If ammonia is detected in the RS at a level less than 1000 ppm, crew members will wait a few hours for the ammonia to diffuse throughout the RS, thus reducing the levels to a safe limit. If the readings show high ammonia levels (1000 ppm), crew members will enter their Soyuz spacecraft and begin cleaning its atmosphere, or prepare for a return to Earth. Since the Soyuz volume is so small, ammonia can be purged using the respirators. This is done by breathing in air through the filters of the respirator, thereby trapping the ammonia. If the Soyuz atmosphere contains too much ammonia (approximately 1200 ppm), the crew will not have enough filters to fully clean the Soyuz air and will have to quickly return to Earth. Note that these procedures will have to be updated when the commercial crew vehicles begin transporting astronauts to the ISS, mainly because the crew vehicles will be docked very close to the likely source of the ammonia—the heat exchangers in Node 2. Since this means the astronauts will have to pass through the highest concentrations of the toxic chemical to get to their escape vehicle, additional O 2 tanks and a scrubber that can remove ammonia from the atmosphere will be stowed on the commercial spacecraft. Conclusion As the heart and lungs of the vehicle, the ECLSS allows crew members to focus on the important research they were tasked to perform. Emergency response is a priority on the ISS, as is maintaining a comfortable, productive environment in which humans can live and work. This effort requires a plethora of systems, subsystems, and experts to monitor and control all the elements necessary to not only survive, but also to thrive in the severe environment of space.