Secondhand smoke exposure is a risk factor for having a heart attack. It's thought that chemicals in secondhand smoke can irritate the lining of your arteries, causing them to swell (inflammation). This inflammation can narrow your arteries, increasing your risk of having a heart attack.
Breathing secondhand smoke can also cause the cells in your blood that are responsible for clotting (platelets) to increase in number, making your blood more likely to clot. Too many platelets can cause a clot to form that may block an artery, causing a heart attack or stroke.
Also, it appears that heart attack rates go down in areas after those areas pass smoking bans. If you smoke, the best way to reduce your heart attack risk is to quit. If you're regularly around smokers, encourage them to quit or smoke in outdoor areas that will reduce the amount of secondhand smoke others will breathe. This is especially important if you have had a previous heart attack or have been diagnosed with heart disease.