I have never seen OSHA incorporate by reference or use specific NIOSH language to support a General Duty Clause violation. Some ANSI or actual industry standards are specifically incorporated by reference within the regulation(s) and in those situations that language is enforceable as a vertical standard when the compliance officer is working to prove prima facie evidence to support the violation. Another consideration are those states with approved state OSHA plans that often times incorporate some very strict language or industry standards that go beyond Federal OSHA standards. All that said, OSHA has been finding more creative ways to issue violations when they are faced with older standards that are not written very clearly. For instance, historically OSHA’s stance regarding the multi-employer clause was not to enforce It within the oil and gas industry. However in 2016 the first multi-employer citation in oil and gas was issued and at least 1 of those employer’s accepted the violation. Great question, hope this provides some food for thought.

NIOSH reccomendations are just that, recommendations. Assuming you mean NIOSH Occupational Exposure Limits? The only way OSHA can cite you for something that is not in a regulation is through the General Duty clause and those citations tend to go away rather quickly if you contest them. That being said, most of the PEL's are still from the 70's and haven't been updated in decades. If NIOSH, AIHA, ACGIH, EPA reccomends a more conservative exposure limit, you should use the most conservative OEL regardless of regulatory requirements.

A bad habit a lot of safety professionals fall into is trying to achieve "compliance" rather than protecting the workers they are responsible for. Compliance is a minimum, especially if you're just going by the FedOSHA standards and not in a more stringent state plan (CalOSHA, Washington DOSH, ect) . If that's all you try to achieve, you're not aiming for excellence.