Basically, pollution is your best bet for localized toxic air. In fact, Asia's got a real world example for you: http://seattletimes.nwsource.com/html/nationworld/2008389439_browncloud14.html

"For those who breathe the toxic mix, the impact can be deadly. Henning Rodhe, a professor of chemical meteorology at Stockholm University, estimates 340,000 people in China and India die each year from cardiovascular and respiratory diseases that can be traced to the emissions from coal-burning factories, diesel trucks and kitchen stoves fueled by firewood."

If some countries in your friend's novel had an even more concentrated toxic mix, not only would more people be dying faster, but the sun-blotting, plant killing effects would probably be worse, too. Which would further wreck the air quality.