What are the factors that determine the level of harm an introduced chemical has on the environment?

why is nonpoint source pollution difficult to control?

1 Chemicals can be toxic because they can harm us when they enter or contact the body. Exposure to a toxic substance such as gasoline can affect your health. Since drinking gasoline can cause burns, vomiting, diarrhea and, in very large amounts, drowsiness or death, it is toxic. Some chemicals are hazardous because of their physical properties: they can explode, burn or react easily with other chemicals. Since gasoline can burn and its vapors can explode, gasoline is also hazardous. A chemical can be toxic, or hazardous, or both.   

2 Non point source water pollution is difficult to control because it comes from the everyday activities of many different people, such as fertilizing a lawn, using a pesticide, or constructing a road or building.