二手菸已被國際癌症研究中心（IARC）歸類為一級致癌物質，全球每年有超過700萬人死於菸害。臺灣每年約有2萬4,000名死於吸菸，而有3,000名死於二手菸害，每年因二手菸罹病約達15萬至23萬人，平均約2至3分鐘就有1名不吸菸者因他人吸菸而罹患疾病。
二手菸是被動或非自願吸入的環境菸煙（Environmental Tobacco Smoke，ETS）。它是分佈最廣且有害的室內空氣污染物，已經被聯合國世界衛生組織列為「頭號的致癌物質」。不吸菸者被迫吸入菸煙，又可稱為被動吸菸或非自願性吸菸。
二手菸是分布最廣且為有害的室內空氣污染物，可說是室內霾害(PM2.5)的主要來源。根據Semple等人2014年刊登於菸草控制(Tobacco Control)期刊的研究報告指出，吸菸者家中的細懸浮微粒(PM2.5)濃度，是非吸菸者家中的10倍。非吸菸者終生與吸菸者同住，其PM2.5暴露濃度，等於住在一個霾害嚴重的城市，但若家中沒有吸菸者，則會降低70%的PM2.5暴露濃度。
此外世界衛生組織指出，與吸菸者同住，得到肺癌的機會比一般民眾高出20%-30%，研究已證實二手菸暴露會增加罹患冠狀動脈心臟病及罹病死亡之風險。然國人對於吸菸可能致癌的觀念相對清楚，對於吸菸與心血管疾病的關係較不熟悉，遑論二手菸會增加心血管疾病風險的認知。
二手菸除會造成或加重幼兒氣喘、支氣管炎、肺炎、中耳炎等呼吸道疾病外，且與兒童白血病、淋巴瘤、大腦與中樞神經系統病變、肝母細胞瘤等癌症有顯著關係；但多數人卻不知道，二手菸對兒童所帶來的新傷害，包括肥胖、身體質量指數(Body Mass Index, BMI)較高、腰圍較大等。
The Hazard of Secondhand Smoke
Secondhand smoke has been classified as Group 1 carcinogen by the International Agency for Research on Cancer (IARC). More than 7 million people have died from tobacco hazards worldwide each year. In Taiwan, approximately 24,000 residents had died from smoking, and 3,000 residents were killed by secondhand smoke. The annual incidence of secondhand smoke-related diseases was about 150,000 to 230,000 people, whereas 1 non-smoker may suffer from secondhand smoke-related diseases every 2 to 3 minutes.
Secondhand smoke refers to the passive or non-voluntary inhalation of environmental tobacco smoke (ETS). It is the most widespread and hazardous indoor air pollutant, which has been tagged as the “top carcinogen” by the WHO. As non-smokers were forced to inhale smoke, secondhand smoke is also called “passive or non-voluntary smoking”.
Secondhand smoke is the most widespread and hazardous indoor air pollutant, which is also the primary source of PM2.5. According to the study that published on the journal (Tobacco Control) in 2014 by Semple et al, the concentration of PM2.5 in the smoker’s house was ten times higher than that in non-smoker’s house. For non-smokers cohabiting with smokers lifelong, the exposure level to PM2.5 equals to living in a smoggy city. However, for those cohabiting with non-smokers, their exposure levels to PM2.5 could be reduced by 70%.
In addition, the WHO data have confirmed that the incidence of lung cancer among persons cohabiting with tobacco users increased by 20%-30% comparing with those cohabiting with non-smokers. Previous studies also demonstrated that the exposure to secondhand smoke increased the risks of coronary heart disease and death. However, our residents may be still unfamiliar with the correlation between smoking and cardiovascular diseases (comparing with the well-known correlations between smoking and cancers), their knowledge about risks to secondhand smoke-induced cardiovascular diseases is far from enough.
Secondhand smoke not only increases or aggravates pediatric asthma, bronchitis, pneumonia, and otitis media, etc., but also is significantly correlated with pediatric leukemia, lymphoma, brain-central nervous system lesions, and hepatoblastoma, etc. But most of the people are not aware of the novel damages to children caused by secondhand smoke including obesity, higher body mass index (BMI), and larger waist size, etc..