When you first turn on a heater after it has been idle, dust and debris that have settled on the heating elements can burn off, creating a temporary smell. If a heater is overheating, it can cause materials inside or around it to burn or emit a smell. This could indicate a problem with the heater or a need for maintenance. In humid environments, heaters can develop mold or mildew, which can produce a musty odor when the heater is in use.
When the Xiaomi heaters are accomplished, there will be a small amount of oil in the manufacturing process of the heating parts, which will cause odor when it is first used due to the sealing during transportation. The odor will gradually disappear after heating in a ventilated place for a period of time, and it will not affect your health, so you can use it with confidence.