Secondhand smoke is cigarette, cigar, or pipe smoke that is inhaled unintentionally by nonsmokers. The amount of such smoke inhaled by a nonsmoker is relatively small when compared to the amount inhaled by a tobacco user; however, so-called passive smoking can aggravate(悪化させる)respiratory illnesses and contribute to serious diseases, including cancer. This danger has led many countries to adopt smoking bans in indoor spaces.