This is a great question and can be answered in a few ways.
We can say that in moderation radiation can be helpful. For example, if you ever broke a bone or had some other ailment and went to the hospital to get a scan, then you have been radiated. However, the conclusion is beneficial, because the doctors will now know what is wrong with you and how they can treat you. So, there are some benefits to humans.
However, radiation in large levels and if it is prolonged, it is very dangerous. The reason for this is because radiation causes ionization to take place and this causes several types of subatomic particles that fly through the air at very high speeds. These, then, hit other cells, even the cells in the human body and this can cause several anomalies to take place. If the levels of radiation are large, a person can die in a few days. If the radiation is prolonged, there can be reproductive disorders, genetic mutations, and cancer. So, yes all radiation in high levels or for a long time is harmful.