From a 1988 study in China, the US protection agency quantified the lifetime exposure of arsenic in drinking water at concentrations of 0.0017 mg/L (1.7 ppb), 0.00017 mg/L, and 0.000017 mg/L are associated with a lifetime skin cancer risk of 1 in 10,000, 1 in 100,000, and 1 in 1,000,000 respectively. WHO asserts that a water level of 0.01 mg/L (10 ppb) poses a risk of 6 in 10000 chance of lifetime skin cancer risk and contends that this level of risk is acceptable.. According to the passage, which substance may pose a risk of skin cancer?