Modern heaters are generally considered safe for humans. In the old days, smoke from gas heaters would leak into the interior of the house and pollute the air. However, if the heater is in good condition, this will not happen with modern heaters. Another problem with heaters is the risk of injury from touching the heating elements directly with the skin. But as there will be a protective cover over the heat exchanger, this will not happen unless one intentionally tries to do so. Another factor to consider is electrical safety. You should only buy a heater from a reputable brand.