The first-aid rule is short, and almost every question about it is really a question about one phrase.
The text. The employer shall ensure the ready availability of medical personnel for advice and consultation on matters of plant health (29 CFR 1910.151(a)). In the absence of an infirmary, clinic, or hospital in near proximity to the workplace which is used for the treatment of all injured employees, a person or persons shall be adequately trained to render first aid; adequate first aid supplies shall be readily available (29 CFR 1910.151(b)). Appendix A to 1910.151 is non-mandatory guidance on kit contents, pointing to the ANSI Z308.1 minimum first aid kit.
What "near proximity" means. The regulation leaves the phrase undefined. OSHA has interpreted it in letters that predate this tool's corpus, framing the question as response time rather than distance: for workplaces where serious injuries such as suffocation, severe bleeding or other life-threatening events are possible, emergency care must be able to reach the employee within a few minutes; where such injuries are not reasonably anticipated, a longer response is acceptable. Those letters are the authority on the point and should be read directly on osha.gov before relying on a specific number.
Reading it the practical way. Two questions decide the duty. First, could a serious injury happen here — in a machine shop, a warehouse with forklifts, a kitchen with fryers, the answer is yes. Second, could outside medical care reach that injured person fast enough, every shift, including nights and weekends? If either answer is doubtful, the conservative reading is to have trained first-aiders on every shift and a stocked kit within reach. Several substance-specific standards add explicit requirements regardless, and construction sites follow the parallel rule in 29 CFR 1926.50(c).
Most employers who think about this end up training someone on each shift. It is cheaper than the argument.