What is a negative externality?

For instance, let's say that a mining company decides to start drilling in the Yountville, CA. The people of Yountville have no impact on this decision. In other words, they have not taken part and did not deiced where the company should drill. The mining company starts to drill and some of the civilians get sick from waste that washed into the water. This is a negative externality because people who had no part in the decision to do something are being negatively effected. In this case, the people of the town had no part in the decision and yet are getting sick from the mining.