If you need an example of a negative externality for your class today then here is a good one from NOAA

From NOAA Research News (One facility makes a big contribution to Salt Lake’s winter brown cloud):

The 2.4 million people who live along Utah’s Wasatch Front experience some of the most severe winter particulate matter air pollution in the nation. Now, analysis of measurements taken during NOAA research flights in 2017 indicates that emissions from a single source, a magnesium refinery, may be responsible for a significant fraction of the fine particles that form  the dense winter brown clouds that hang over Salt Lake City. 

The finding was published this week in the journal Environmental Science and Technology.

This diagram depicts the estimated influence of halogen emissions on the generation of PM 2.5 by halogens, including chlorine and bromine, based on measurements during NOAA research flights over the Salt Lake basin in Utah in January - February 2017. Credit: Caroline Womack, NOAA/CIRES

Lead author Carrie Womack, a CIRES scientist working at NOAA, said analysis of airborne measurements directly from the plume rising from the US Magnesium refinery during a 2017 winter air pollution study in Utah found that emissions of chlorine and bromine, known as halogenated compounds, were significant contributors to the persistent winter brown clouds. 

And here is a great picture of a negative externality if your students don't believe you: