Small amounts of radiation can cause very little harm to the human body. Scientific studies have shown that rays below 100mSv do not cause significant harm to the human body, but once the dose of X-rays exceeds 100mSv, it may cause harm to the human body. The radiation dose for a single DR examination is about 0.3-0.5mSv and for a chest CT examination is about 3-5mSv, so within the normal permissible safety range, a patient can receive up to 200 chest DR examinations or 20 chest CT examinations in a year. Once this range is exceeded, there is a risk of harm to the human body.