It seems clear that air pollution not only causes increases in heart attacks and strokes, but also more heart arrhythmias and heart failure exacerbations.
Air pollution is comprised of gases (carbon monoxide, ozone, sulfur dioxide, oxides of nitrogen), as well as particulate matter that is small enough to be inhaled (from motor vehicle emissions, tire fragmentation, smelting and other metal processing, power generation, wood burning, pollen, and mold, etc.).
These substances can cause heart issues by increasing the propensity for blood clots to form, making heart arrhythmias more likely, causing vasoconstriction, increased inflammation, and accelerated atherosclerosis. This is similar to the well-known risk of second hand smoke - being in a home or workplace with cigarette smokers.
Rabbits with high cholesterol who are exposed to increased concentration of particulate matter showed significant increased plaque build-up in their arteries, compared to the group not exposed.
There seems to be a linear relationship between the amount of particulate pollution and cardiac risk, with no safe threshold.
Air pollution is not good for your heart (or your lungs). The question then becomes, what can you do about it? You can adjust your other risk factors by not smoking, eating right, exercising, and keeping your cholesterol levels low. One can stay inside on particularly bad days or move to a "cleaner" environment. You can work with legislators and industry to reduce the amount of pollution being put into our atmosphere.
Much progress has been made, more is needed.