Which element has a Primary MCL set at 4 mg/l due to a direct threat to human health?

The element with a Primary Maximum Contaminant Level (MCL) set at 4 mg/l due to a direct threat to human health is arsenic. Arsenic is a naturally occurring element that can contaminate groundwater supplies, primarily through natural processes and, in some cases, human activities. The health risks associated with arsenic exposure include cancer and skin lesions, and long-term exposure can lead to developmental effects, cardiovascular disease, neurotoxicity, and diabetes. Because of these serious health impacts, regulatory agencies, such as the Environmental Protection Agency (EPA), have established a stringent MCL to protect public health.

In contrast, while nitrate can pose health risks, particularly in infants, its MCL is set at a different level, and its risks are related to different health issues. Chlorine is commonly used as a disinfectant in water treatment and does not have a health-based MCL at the same level for its presence in treated drinking water. Fluoride, while regulated, is primarily monitored for dental health benefits and is set at a different health-based standard. Thus, arsenic stands out as the element specifically regulated at the mentioned MCL for direct health threats.