Second-hand smoke ( SHS ) , a cocktail of toxic chemicals , cannot be ignored

Tobacco use is the leading cause of preventable diseases and premature deaths worldwide . Its use killed 100 million people in the 20th century and if current trends continue, tobacco will kill one billion people in 21st century, with 80 percent of these deaths from low and middle income countries . Despite, the massive death toll, almost a billion men and 250 million women smoke in the world. Half of the male smokers live in low and middle income countries . However, smokers don’t pay alone; the unfortunate victims who unnecessarily suffer are the secondhand smokers.


Active and secondhand smoking burden
Tobacco use is the leading cause of preventable diseases and premature deaths worldwide [1] .Its use killed 100 million people in the 20th century and if current trends continue, tobacco will kill one billion people in 21st century, with 80 percent of these deaths from low -and middle -income countries [2] .Despite, the massive death toll, almost a billion men and 250 million women smoke in the world.Half of the male smokers live in low and middle -income countries [3] .However, smokers don't pay alone; the unfortunate victims who unnecessarily suffer are the secondhand smokers.
A landmark and extensive study that sensitized the world about the harmful effects of the secondhand smoke (SHS) was by Mattias Oberg and colleagues [4] .They presented the first estimates on the extent of worldwide exposure to SHS in 2004 and the associated burden of disease and premature deaths.It was concluded that number of people exposed to SHS and the associated death toll is very high.Among non-smokers who were exposed to SHS, 40% were children, 35% women and 33% men and SHS caused 603 000 deaths worldwide according to the estimates which covered 192 countries by the authors [4] .The established findings since mid-1980s about the links between SHS and specific diseases by different scientific communities like International Agency for Research on Cancer (IARC), World Health Organization (WHO) [5] , the California Environmental Protection Agency [6] , and the US Surgeon General [7] confirmed the adverse health effects of SHS.However, (IARC) confirmed that SHS is carcinogenic to human in 2004 [8] and banned smoking at workplaces in 2009 [9] .The associated effects of SHS include heart disease, worsening of asthma, sudden infant death syndrome, cancer and many more [7] .

Relative toxicity of direct and indirect tobacco smoking
Tobacco smoke contains over 4000 chemicals, including more than 69 well known carcinogens [8,10] .However the level of toxicity is considered higher in secondhand smoking than in active smoking.Secondhand smoking-the inhalation of a mixture of smoke from sidestream and exhaled mainstream which is inhaled by secondhand smoker as well as by the active smoker itself [7,11] .The smoke emitted from the tip of a cigarette has about double the concentration of nicotine and tar than the smoke being inhaled directly by the smoker.It also contains about three times the amount of the carcinogen benzo (a) pyrene, five times the level of carbon monoxide and about 50 times the amount of ammonia [8] .The sidestream smoke (SSS) is more harmful as the COMMENTARY concentration of other toxic chemicals which include, polynuclear aromatic hydrocarbons, tobacco-specific N-nitrosamines, volatile N-nitrosamines and aromatic amines (4-aminobiphenly) is much higher [12] .The load of toxic chemicals further increase when the SSS mixes with the exhaled mainstream smoke which is inhaled by both smokers and non-smokers [13][14][15][16][17][18][19] .In addition the vapor-phase particulates from SSS have been reported to be absorbed more quickly into blood and lymph systems than particulate phase constituents found in mainstream smoke [19] .

SHS and cancer
Overall, tobacco use, accounts to be the cause of about 30% of cancer deaths [20] .Numerous epidemiological studies have reported causal relationship between tobacco use and the development of different cancers including esophageal squamous cell carcinoma (ESCC) [21][22][23] .No doubt, studies have associated secondhand smoking with cancers of the lung, larynx, and pharynx, however, the association between secondhand smoking and ESCC has not been investigated to the extent as studied with active smoking [8] .Only a few studies have examined the association between secondhand smoking and ESCC and the findings are inconclusive.This indicates the need of epidemiological studies to investigate the public health concern, implications of tobacco use in the form of secondhand smoking and its consequences including ESCC risk.The share and contribution of tobacco smoking in ESCC development and mortality is likely to increase further in the developing countries as its consumption is worryingly increasing, [24,25] and if such smoking patterns persist an epidemic of cancer attributed to tobacco smoke inhaled by active or secondhand smokers is expected to occur in developing countries [26,27] .Hence, it is important to understand the role of secondhand smoking further in ESCC development in these high ESCC risk regions where the use of tobacco is on rise.

SHS and ESCC
In the recent study published in Medicine [28] , the association between secondhand smoking and risk of ESCC was estimated in Kashmir, where the incidence of ESCC is high and is a part of "Asian esophageal cancer belt".Numbers of factors are associated with ESCC in Kashmir [28][29][30][31][32][33] but etiology still remains to be understood completely.Additionally Kashmir valley has a unique socio-cultural setting and living in joint families is a characteristic feature.People sit together usually in kitchens where elders enjoy traditional habit of sharing hookah for hours.Majority of the people are from rural backgrounds where adobe house are common [29] and the SHS exposure can get aggravated in such dwellings.Hence Kashmiri culture and high risk of ESCC offers a suitable setting to study the effect of SHS on ESCC.
The study concluded with an indication of increased risk of ESCC associated with exposure to SHS.Although the observed associations were not statically significant, our results suggest a dose-response borderline significant association for SHS exposure >14 hours per week.Further, the SHS exposure was more common in participants who lacked formal education or were poorer.In addition, relatively 'low intensity' of active smoking and relatively modest number of people with SHS exposure, might be the reasons for not observing statistically significant associations in this study.However, more studies with a larger sample size are required before making any conclusion on the association between secondhand smoking and ESCC risk.Nevertheless, the earlier studies on ESCC either have not stratified the participants into tobacco users and never tobacco users or have not taken into account the possible confounding effects of other risk factors of ESCC.To the best of our knowledge, this was the first study to report increased risk of ESCC due to SHS exposure in dose dependent manner.Overall, this work adds profoundly to the growing literature of secondhand smoking and its association with ESCC risk.In a larger picture our results may help to increase the awareness about harms of secondhand smoking, particularly in developing populations where ESCC is a major health concern and tobacco use is on rise.

Impact on tobacco free world policy
Despite, of the innumerable adverse health effects of SHS, a common indoor air pollutant, the comprehensive smoke free laws and the enforcement of these laws is present in the jurisdictions of only 7.4% of population worldwide [34] , while as, almost 93% of the world's population is still living in countries not covered by fully smoke-free public health regulations [35,36] .In the world's first major public health treaty, encompassing 172 nations "Framework Convention on Tobacco Control" (FCTC) since 2003, only 60 countries have countersigned to FCTC and have initiated campaigns for smoke-free laws and more than 17 countries have a national law requiring all workplaces and public places to be smoke-free [35] .Unfortunately, the vast majority of the globe especially the developing countries remains without protection from SHS exposure due to inadequate political implementation of these evidence-based policies, often spurred by tobacco industry opposition.Therefore, the immediate message based on the global profile of exposure to SHS and disease burden highlights that the time has come to act seriously to control tobacco at local and global level to make "tobacco free world" and save lives especially in poor countries suffering from dual burden of both communicable and non-communicable diseases.
Therefore, more studies covering different cancers and nations needs to be carried out to assess the effects of SHS as well as its impact on the various policies for reduction of SHS exposures.