They can be very harmful but its like saying how harmful is heat ?
If you are exposed to infrared rays for a long period of time they can damage your vision and cause burns. This all depends on how hot they are ect. Infrared rays are not as harmful as X-Rays and Gamma Rays as these can both produce cancer cells, if exposed to often. That is why doctors and nurses have to stand behind a glass protector so they are not exposed to the X-Rays.
If you are exposed to infrared rays for a long period of time they can damage your vision and cause burns. This all depends on how hot they are ect. Infrared rays are not as harmful as X-Rays and Gamma Rays as these can both produce cancer cells, if exposed to often. That is why doctors and nurses have to stand behind a glass protector so they are not exposed to the X-Rays.