Potentially dangerous situation or controlled risk.
Are nanoparticles dangerous? This issue is being investigated very actively. Without the appropriate information, controlling exposures can limit the levels of risks, as a precaution.
Risk definition
How is it possible to define a risk?
An isolated shark in the ocean is potentially dangerous but is not a risk. When a swimmer is nearby, he is exposed to a level of risk. If he is bleeding due to an injury, the level of risk increases. If he is protected by a cage, the level of risk decreases.
A welding machine is potentially dangerous. A welder, when protected with the appropriate equipment, takes a controlled risk. Without protection, the risk to his vision is important. So, risk is function of danger AND exposure: RISK = f ( DANGER, EXPOSURE).