According to the United States Environmental Protection Agency, "radioactive antiques [including watches] are usually not a health risk as long as they are intact and in good condition." However, radium is highly radioactive, emitting alpha, beta, and gamma radiation — the effects of which are particularly deleterious if inhaled or ingested since there is no shielding within the body. Indeed, the body treats radium as it does calcium, storing it in bone where it may cause bone degeneration and cancer.
Therefore, it is of the utmost importance that watches with radium dials should not be taken apart without proper training, technique, and facilities. Radium paint can be ingested by inhaling flaking paint particles. The alpha particles emitted by the radium, which is taken up in bone, will kill off surrounding bone tissue, resulting in a condition loosely referred to as radium jaw. Inhaled or ingested particles may deposit a high local dose with a risk of radiation-caused lung or gastrointestinal cancer. Penetrating gamma radiation produced by some dials also represents a significant health risk.
Although old radium dials generally no longer produce light, this is due to the breakdown of the crystal structure of the luminous zinc sulfide rather than the radioactive decay of the radium. The radium isotope (Ra) used has a half-life of about 1,600 years, so radium dials remain essentially just as radioactive as when originally painted 50 or 100 years ago, whether or not they remain luminous.
Radium dials held near the face have been shown to produce radiation doses in excess of 10 µSv / hour. After about 20 minutes this delivers the equivalent of one whole day's worth of normal background radiation. This rate probably only represents the dose rate from gamma emission, as the alpha emission will be stopped by the lacquer, or crystal, or case; hence, the dose rate following ingestion or inhalation of the dust could be much higher.
Chronic exposure to high levels of radium can result in an increased incidence of bone, liver, or breast cancer. Decaying radium also produces the gas radon, recognized as the second leading cause of lung cancer in the United States and the United Kingdom. A 2018 study by researchers from the University of Northampton found that a collection of 30 vintage military watches with radium dials kept in a small, unventilated room produced a radon concentration 134 times greater than the UK's recommended "safe" level.
Ingestion of radium has been linked to anemia, cataracts, broken teeth, and reduced bone growth.