Which contaminant poses the greatest health risk at a concentration of 0.005 mg/L?

• A. Benzene
• B. Lead
• C. Mercury
• D. Nitrate

Answer: (A)

In drinking-water health risk assessment, the danger of a contaminant at a given concentration depends on how toxic it is and what levels are considered acceptable. Benzene is a very potent carcinogen with a very low allowable concentration in drinking water (about 5 micrograms per liter). So a measured level of 0.005 mg/L equals that limit, meaning that even small exposures over time carry a real cancer risk. Lead and mercury are indeed toxic, but at this concentration lead is below its typical action-level threshold and nitrate is far less hazardous at such a level. Mercury has a very strict standard as well, but the key point here is benzene’s high carcinogenic potency and its tight regulatory limit, which makes it the greatest health risk at this exact concentration.