Why Diesel Cars Are Spoiling Your Summer

City dwellers may be coughing and wheezing more, too. Diesel vehicles, which still command nearly half the market for new cars, are left with barely any pollution controls when temperatures soar above 35 degrees Celsius (95 Fahrenheit), according to France’s Petroleum and New Energies Research Institute. “There are higher emissions of nitrogen oxides because the setups don’t work as well when it’s very hot,” said Gaetan Monnier, who heads the unit that led random government probes of cars after the VW scandal.