What is the steepest angle a ramp can be?

Could you please explain to me what the maximum angle a ramp can have before it becomes too steep for SAFE use? I'm interested in understanding the factors that determine this limit, such as the weight of the objects being transported and the safety concerns for users. Additionally, I'd like to know if there are any industry standards or regulations that dictate the steepest angle a ramp can safely be for different types of applications.