Regulations on accelerated or longterm stability testing mostly require humidity to be controlled within a tolerance of ±5%RH. If the condition of the atmosphere falls outside this window, it can result in a test having to be restarted. This is expensive and can even delay the launch of a new product.
Many cleanroom manufacturing processes require control over humidity. For instance, the printing of diagnostic strips using water-based inks can require a high humidity of between 95-98%RH, ±1%RH. In such circumstances, when the optimum humidity is so precise, it is often not possible to maintain a whole room at such a close level so a contained micro-climate can be produced around an individual manufacturing process.