Human factors concentrates on the interfaces between the human (the ‘L’ in the center box) and the other elements of the SHELL model, and - from a safety viewpoint - where these elements can be deficient, e.g.:
S: misinterpretation of procedures, badly written manuals, poorly designed checklists, untested or difficult to use computer software
H: not enough tools, inappropriate equipment, poor aircraft design for maintainability
E: uncomfortable workplace, inadequate hangar space, extreme temperatures, excessive noise, poor lighting
L: relationships with other people, shortage of manpower, lack of supervision, lack of support from managers