From smoke and its particles proceeding from combustion vehicles and industry, there is a special group called PM2.5 due to their lesser size than 2.5 microns. They are called ultrafine particles and are thought to cause health issues.
A study asseverates that an increase of five percent in a cubic meter is responsible for a 7% increase in natural deaths. With such a small size these particles can pass to the bloodstream after being inhaled through pulmonary system, so there's no barrier in some situations.
Analyzing this phenomenon, the next conjecture is an association toward cardiovascular disorders. Another common problem appears when the organism doesn't expel the toxicity, but end up in areas where a kind of longtime storage occurs like articulations or bones in general.
Like always there are different opinions about maximum levels of ultrafine particles, with zones stating 10 mcg/m3 as top limit like World Health Organization. The surprise appears when other areas present a superior limit, thus if the aforementioned study is right that would mean more "natural deaths".
Leaving ultrafine particles, air pollution was already dangerous enough to be worried since its potential content of carbon monoxide, partially burnt hydrocarbons, ... the first one is proven lethal in certain proportions. Carbon and hydrogen take oxygen to create new molecules after combustion, so they are strictly related to the oxygen level that we need for our respiration and health.