You don't often see a nice simple linear regression nowadays but here's one.
The study looks at life expectancy in counties of the USA to see the effect of air pollution. The authors assembled data on life expectancy and on air pollution in 211 counties of the USA at two time periods - around 1980 and around 2000. In general life expectancy improved over the 20 years, as did air pollution.
Of course other things might account for the change in life expectancy. People got wealthier. Smoking decreased. In the USA black people are known to have lower life expectancy than white people, so a large influx or outflow of black people from a particular county might have accounted for some of its change in life expectancy. Measures of these are included in the regression models.
The measure for smoking is interesting. The researchers couldn't get actual smoking data for the 211 counties for the two time periods, so they used a proxy - mortality from lung cancer and from COPD. That seems sensible but lung cancer and COPD arefairly major causes of death so using them as predictors of life expectancy (which is just a summary of mortality rates) seems rather incestuous to me. This is the NEJM so I guess the statistical referees would have complained if it was a serious problem.