NTSB issued 2 safety recommendations to physically separate lithium batteries from other flammable hazardous materials stowed on cargo aircraft and to establish maximum loading density requirements that restrict quantities of lithium batteries and flammable hazardous materials. These safety recommendations, addressed to the Pipeline and Hazardous Materials Safety Administration, are derived from the investigation of the July 28, 2011, in-flight fire and crash of Asiana Airlines Flight 991.