Smoking, a cause of multisystem disease, can be considered to be a chronic, progressive and contagious disease. It is one of the major public health problems of today since it is the most important cause of preventable deaths in the whole world. Due to its repercussions on the health and wellbeing of both active or passive smokers and nonsmokers, the act of smoking causes problems for society as a whole because of the large number of persons directly or indirectly reached by smoke. Among them are children who having no freedom of choice, ultimately become compulsory passive smokers since their conception [1] .

Since the introduction of tobacco in Europe in the 15^th century, the smoking habit has been socially accepted and, on some occasions, greatly encouraged, ultimately leading to a constantly increasing number of people consuming cigarettes and consequently being exposed to all the elements originating from their combustion. The current estimate is that more than 1 billion people are smokers in the whole world (80% of them males) and that each day more than 80,000 persons, most of whom are young people, start smoking, especially in countries of lower socioeconomic level [2] . This huge contingent of smokers disseminates the products of cigarette burning to the rest of the world population. At present, approximately 30% of the world’s population is exposed to se- condhand smoke [3] , and it has been estimated that more than 700 million children are exposed to cigarette smoke, with the consequent occurrence of 5 million deaths per year, and a projected estimate that about 1 billion people will die during the 21st century because of smoking, most of them due to respiratory diseases [4] . Most of the children are forced to inhale air contaminated by tobacco smoke in many places like playgrounds, schools, kindergartens and vehicles. England, every year, over 160000 children are adversely affected by tobacco smok- ing costing more than £ 23 million in health care [1] [3] [5] .

The harmful effects of passive smoking on health have been known for more than 80 years. However, the in- terest in this topic has increased since 1967 when these effects were assessed in children [3] [6] [7] . Passive smoking is defined as involuntary exposure of nonsmoking individuals to the substances produced by tobacco combustion indoors. This expression was first used in 1939 and since then this topic has attracted increasing in- terest on the part of investigators because of its importance for public health [7] .

Children, as growing and developing human beings, are extremely vulnerable to the harmful effects of passive smoking because of the following peculiarities: a) they spend more time of their life in the presence of their parents, caregivers and other adults, thus being unable to avoid exposure; b) they are unable to assess the risks present in the environment; c) they have immature and rapidly developing systems (respiratory, nervous and immunological systems); d) have a greater ventilation rate than adults; e) they receive proportionally greater doses during exposure because they inhale more polluting elements per body weight; f) low metabolic capacity; g) hand-to-mouth contact; longer life expectancy [8] [9] .

This is a dynamic, heterogeneous and complex mixture consisting of more than 4700 chemical substances in the form of gases and particles that are generated during the burning of products derived from tobacco and from cigarette paper, which undergo dehydrogenation, oxidation, condensation, and rearrangements. All these ele- ments cause inflammation and damage to cells. Tobacco smoke can migrate though windows, ventilation sys- tems, walls, etc and can to affect people at several places far from the smoking local [10] . Indoor environmental tobacco smoke concentration depends of the number of cigarettes smoked in a period of time, the ventilation rate, the volume of the environment and other process that eliminate pollutants from de air [11] .

For didactic purposes, cigarette smoke can be divided into:

1) First-hand smoke, i.e., the smoke inhaled by a smoker, generated by the combustion of organic elements present in tobacco derivatives, which undergoes variations according to temperature, characteristics of the to- bacco leaves, and the quantity of oxygen in the air [12] .

2) Second-hand smoke: a mixture of the smoke emitted from the burning cigarette and the stream smoke ex- haled from the lungs of the smokers. It is the smoke inhaled by non-smokers. Secondary stream (75%): it is the one originating in the tip of the lighted cigarette associated with the smoke exhaled by the smoker, i.e., the pas- sive cigarette combustion between puffs, which may contain higher concentrations of toxic substances than the main stream (tar, nicotine, carbon monoxide, and nitric oxide) [13] .

3) Third-hand smoke: a pool of pollutants that remain on surfaces and in dust after tobacco has been smoked, re-emitted into the gas-phase or reacting with other compounds in the environment to form secondary pollutants that includes invisible smoke left in the air after a cigarette is extinguished. This smoke deposits on clothing, hair, furniture, curtains, carpet, toys and others surfaces, where it remains for several weeks. People may then come in contact with harmful elements such as phenols, cresols, formaldehyde, nicotine, napthalene, 3-ethinyl- pyridine and tobacco-specific nitrosamine by inhalation or ingestion and through the skin [14] -[17] .

Cigarette smoke can also be divided into two phases according to its components. The gas phase (60%) mainly consists of nitrogen (73%), oxygen (10%), carbon dioxide (9.5%) and carbon monoxide (4.2%), also containing nitrogen oxides, nitrosamines, acetaldehyde, hydrazine, hydrocyanic acid, ammonia, vinylchloride, and acroleid. Environmental smoke tobacco causes effects on the respiratory system within the first few minutes to smoke exposure and leads to decrease in forced expiratory volume in 1 second (FEV1) and in FEV1/forced vital capacity (FVC) ratio [18] . In turn, the particulate phase contains particles measuring on average 0.6 µm in diameter, a fact that favors their deposition in the upper airways, as well as other harmful elements such as nico- tine, 4-aminobiphenyl, cresol, polynucelar hydrocarbonpehnol, carbazole, catechol, and indole [19] .

Cigarette burning produces thousands of aggressive substances affecting the organism either by a direct action on tissues, interfering with enzymatic reactions by being deposited, or indirectly, reacting with other environ- mental elements.

Nicotine is a volatile colorless base extracted from tobacco leaves which acquires a brownish color when ex- posed to the air. It is classified as a psychoactive substance because of its action on the parasympathetic and sympathetics systems, increasing adrenaline discharge by the adrenal glands [20] and thus representing a strong- ly addictive substance. People exposed to high levels of environmental tobacco smoke may absorb amounts of nico- tine comparable with light or non-daily smoking [21] .

Nicotine is well absorbed by the oral membranes, the respiratory tract, the gastrointestinal tract, the bladder, and the skin. Since it weakly binds to proteins, it can be distributed throughout all tissues, also crossing the pla- cental barrier and being excreted through maternal milk [22] . Nicotine is metabolized by the liver (90%), lungs, kidneys, brain, and respiratory epithelium. It acts as a mood stabilizer, increasing the ability to concentrate and intellectual performance, also having a chronic antidepressant effect leading to induced consumption by users. When inhaled, its effects on the central nervous system can be observed within less than 7 seconds. In maternal milk, nicotine has a half-life of 90 minutes and its concentration can reach 0.5 mg/liter among women who smoke more than 20 cigarettes per day, since women have a lower capacity to metabolize nicotine than men.

Besides being an important teratogen, nicotine causes changes in the structure and functioning of the organs, thus increasing the susceptibility to diseases, and also reacts with oxidant gases present in the environment and gives origin to nitrosamines and nitrosorcotinine, which are pulmonary carcinogens [23] . The main changes caused by nicotine in the organism are: nauseas, vomiting, diarrhea, changes in heart rate, increased arterial pres- sure, tremors, peripheral vasoconstriction, respiratory excitation, bronchoconstriction, increased bronchial and salivary secretion, and convulsions. Children display higher relative nicotine levels during and after exposure to environmental tobacco smoke than adults [24] .

This is a not specific gas tobacco. This is one of the various gases produced by a burning cigarette which pro- duces 20 to 60 thousand ppm, when the acceptable standard for good quality air is up to 9 ppm. It is a colorless, odorless, tasteless and toxic gas formed by the combustion of organic matter. In combination with nicotine, it is one of the major factors responsible for the acute toxicity of tobacco. It is a teratogenic element rapidly absorbed in the blood and can cause direct tissue damage or tissue hypoxia because of its high affinity for hemoglobin, giving origin to carboxyhemoglobin and leading to a reduced oxygen release to tissues [25] [26] .

Smoke contains more than 60 substances with a confirmed cancerigenous action, and the main ones are: benzo- pyrene, dibenzoanthracene, nitrosamines, formaldehyde, acetaldehyde, tar, dibenzoacridine, arsenic, polonium 210, cadmium, nickel, toluene, xylene, benzophenanthrene, dibenzefluorethane and levulinic acid with direct or indirect actions on the cells, reinforcing the enzymatic activities and/or having an additive action with other en- vironmental pollutants.

A child is exposed to smoking since its conception, being submitted to the harmful effects of all the components detected in the burning cigarette. The main effects on the child’s organism can be divided into the following groups:

The harmful effects of smoking have been long known and little has been done so far to avoid their conse- quences for health. It is known that there is no safe level of exposure to tobacco smoke and that millions of children continue to be dangerously exposed to passive smoking [55] . Every person, and especially every child, has the right to grow and live in healthy, clean and safe environments, protected from the effects of passive smoking [56] . The most effective means of protecting children from passive smoke is to reduce de prevalence of smoking in adults. Banning smoking in public places outside of the home, including workplaces, restaurants and bars leading to improved air quality in those locations [10] [57] , because elimination of exposure to second-hand smoke is associated with immediate positive effect on respiratory function [58] -[60] . Combating these effects should be one of the priorities of public health, accepted as a moral duty of society as a whole.