Unfortunately, the answer may be "in Abuja" (the capital of Nigeria). The most useful document is a listing of the 24 volumes of The Laws of Nigeria 1990 here (jost a chapter list), and there is no doubt a printed copy of the entire collection in Nigeria. Probably there is a copy elsewhere, but that may not help you (e.g. if it is in the law school library at Yale). As you can see, there are more gaps than links in the online coverage. However, the Labour Act (Volume X, chapter 198) is in fact available. The issue would be that there could be relevant laws in another chapter which is not online: but, this is probably good as a first approximation. The next complication is that this is the 1990 consolidation, and unlike the US, they do not have annual consolidations into a unified code. There is a compilation as of 2000 outlined here, made by the International Centre for Nigerian Law which while no doubt accurate is not authoritative. You can see a list of links about acts / decrees according to year, here: whatever the situation is as reported for 1990, to get the current law you would have to check all of the subsequent acts and decrees. Their list stops as of 2006, and I believe that what they list is only a part of what was enacted in each of those years.
Now, if that wasn't bad enough, each of the 36 states can have their own laws. Plus, as a common law system, you have to consider not just the acts and decrees promulgated by the legislature or military, but also the court decisions (which essentially create additional law, under the guise of "interpreting" acts and decrees). Fortunately, you probably don't need to deal with the fact that they also have four distinct legal systems (English law, common law, traditional law and Sharia). Because Nigeria was an English colony, all English law enacted before October 1, 1960 also holds (unless repealed in Nigeria).
Article 66 of the Labour Act is entitled Labour health areas, and indicates that the (Labour) minister may promulgate regulations...that means you would also need to look at the regulations, and good luck there. This source claims that the Labour Act was revised in 2004, but their link is to the Labour Act 2004 of Namibia (".na" and ".ng" can be confusing).