Complex sentence: In 2001, the most commonly used QLFT was irritant smoke and saccharin, but in 2004, the National Institute for Occupational Safety and Health (NIOSH) advised that fit tester stop using irritant smoke.
Split into simpler sentences: In 2001, the most commonly used QLFT is irritant smoke and saccharin. But in 2004, National Institute for Occupational Safety and Health (NIOSH) advised to stop using irritant smoke.