Lead, a heavy metal, accumulates in the body. Children are especially susceptible to airborne lead. Iron, also a metal, can be problematic in large quantities, but is also necessary in extremely small amounts for proper function.

Fluoride in high doses is extremely problematic, making it a concerning pollutant. It is extremely prevalent in products used in dental health. Because these are products that are generally washed away, fluoride is of special concern as a water pollutant.

Nitrates, a form of nitrogen, are a severe problem in drinking water that has been contaminated with fertilizers. Sewage and fertilizer often go hand in hand from agricultural areas. Arsenic and chlorine are concerning pollutants, but generally not present due to fertilizer use.

Skull and crossbones indicates poison. Corrosive materials are labelled "corrosive" and signs indicating the substance corroding metal may be present. Flammable materials are marked with flames.

Only 5% of carbon monoxide in cities does not originate from car exhaust. Though industrial facilities are a major concern as a source of pollution, regarding carbon monoxide specifically, automobiles have a much higher impact.

Rachel Carson's novel Silent Spring brought widespread attention to the damage of bioaccumulation in ecosystems. Her outcry against the dangerous chemical DDT (dichlorophenyltrichloroethane) was the first to display the issues associated with large-scale pesticide use. Al Gore's work concerned global warming and the other answer choices are more associated with historical events rather than environmental ones. 

Acute exposure is an exposure to a substance over a short period of time in a relatively high concentration. Conversely, chronic exposure occurs over long periods of time at relatively lower concentrations. Toxicity and hazard depend of the circumstance of the substance, not the type of exposure. Acute exposure may or may not be more hazardous or toxic than chronic exposure to the same substance. 

The correct response is Clean Air Act. The signing of this act set a precedent that allowed for the signing of additional environmental acts. This act banned toxic chlorofluorocarbons that had caused the whole in the ozone layer. The Clean Air Act gave the EPA permission to regulate, establish laws, and fine individuals and businesses for violating the act.

Coal deposits often contain high levels of sulfur compounds that are emitted into the atmosphere when combusted. Much of this could be abated with clean coal technology.

NRCS (National Resource Conservation Service) and EPA (Environmental Protection Agency) are federal agencies, not pieces of legislation. NEPA (National Environmental Policy Act) requires environmental assesments for proposed federal projects and RCRA (Resource Conservation and Recovery Act) classifies waste as solid or hazardous and sets regulations for storage and treatment of said waste. CERCLA (Comprehensive Environmental Response Compensation and Liability Act) establishes a superfund for heavily polluted areas and holds responsible firms financially accountable.