Final answer:
To ascertain which carcinogen presents the greatest risk, we look at their residence times. Over the span of a week, Chemical Y poses the highest risk due to its longer residence time in the environment, despite all chemicals being equally harmful at equal concentrations.
Step-by-step explanation:
The threshold limits mentioned refer to the Occupational Safety and Health Administration's (OSHA) permissible exposure limits, which indeed set forth the maximum concentration of hazardous chemicals that workers can be exposed to during a typical eight-hour workday. However, regarding the substances released into the river following an industrial accident, the risk to the nearby community does not only depend on the inherent toxicity of the chemicals involved but also on their residence times in the environment.
To assess which chemical poses the greatest risk of exposure over the course of a week following the spill, we must consider that all three carcinogens are equally harmful at equal concentrations. Chemical X has a residence time of 2.8 days, Chemical Y has 3.5 days, and Chemical Z has 17.2 hours. Given these residence times, Chemical X will linger in the environment for the shortest duration, while Chemical Y will remain a bit longer, and Chemical Z will be present for even less time when converted into days (approximately 0.72 days). Therefore, over the course of a week, Chemical Y would present the greatest risk of exposure to the nearby community due to its longer residence time, assuming the other conditions affecting their dispersion and dilution in the river are similar for all three chemicals.