Is the timing right?
Are we looking for the health outcome at the right time based on when exposure occurred?
The period of time between exposure and the appearance of a related health outcome is known as the "latency period."
Some health outcomes take a long time to develop. If we look too soon after the exposure, we won't find anything, even if the contaminant could in fact cause the health outcome in the future. On the other hand, some health outcomes may only be seen while exposure is going on. In this case, if exposure has ceased, there would be no reason to conduct a study.