Indoor environment temperature is one of the most important parameters influencing workers’ health and productivity. High temperature and low humidity have been associated with respiratory and cutaneous symptoms, such as airway, eye and skin dryness, but also with more general symptoms (headache). As a matter of fact, low humidity and high ventilation have been associated with a quicker evaporation of the lacrimal film in indoor workers [7]. On the contrary, a too high humidity seems to increase discomfort in indoor working environments because of an increased release of bad smell from materials contaminated by moulds. In fact high humidity could increase the growth of fungi and dust mites, which could affect the health and wellness of workers staying indoor [8]. Nevertheless, temperature and humidity could influence also the concentration of chemicals in indoor air, especially particulate matter and chemicals released by furniture, insulation material and carpets. This would make difficult to understand whether symptoms related to indoor air are associated more with physical pollutants or with the influence of physical parameters on chemical composition of indoor air.

Radon is a natural radioactive carcinogenic compound, which is usually found in indoor air. It derives from the natural release from the soil where the building is placed. Radon high-releasing soils are volcanic rocks and materials (tuff, pozzolan, etc.) and granites. Radon is usually concentrated in lower floors, especially those underground, and its contamination is fostered by the natural flow between outside cold air and inside warmer air (the so-called chimney effect); ventilation is also an important factor which usually reduces the concentration of radon in indoor tight environments. Radon is a known carcinogenic compound for the International Agency for the Research on Cancer (IARC), and many studies have issued the problem of indoor air contamination in occupational and nonoccupational settings [9].

Formaldehyde is found in building materials and furniture (panels, plywood, chipboard, medium-density fibreboard (MDF)), glues, insulating foams, resins and environmental tobacco smoke. Formaldehyde in indoor air could come also from the reaction between ozone and other VOCs or aliphatic hydrocarbon (from printers) or alkanes (from cleaning products and air fresheners) [10]. The amount of formaldehyde released in the environments is higher for newly constructed living and working environments, including newly produced furniture. Moreover, formaldehyde concentration is influenced by temperature and humidity of indoor air. In offices formaldehyde concentration is around 0.045 mg/m³; for these concentrations some worker could start to perceive formaldehyde smell as well to report various symptoms such as nose, throat, eyes and skin irritation, headache, sleepiness and dizziness [11]. Some studies highlighted a possible role of formaldehyde in causing asthma and rhinitis, by the interaction with other chemicals (ozone, VOCs) and directly enhancing the expression of adhesion molecules on nasal mucosa [12].The IARC has classified formaldehyde carcinogenic for human beings, because of its role in causing nasal cancer and other malignancies [13].

VOCs are organic compounds found in indoor air, with a boiling point between 50 and 250 °C. They are rather many comprise of aliphatic and aromatic hydrocarbons, cycloalkanes, aldehydes, terpenes, alcohols, esters and ketones [14]. Major sources of VOCs are building materials, paints, glues, furnitures, cleaning products, printers and photocopiers. VOCs are also produced by chemical reaction between other chemicals, especially ozone and nitrous compounds, and by moulds (the so-called MVOCs), being responsible of the bad smell often perceived in indoor environments. Exposure to VOCs has been associated with airway irritation, inflammation and obstruction and general symptoms (sleepiness, dizziness), even if this finding has not been always confirmed [15].

Ozone concentration in indoor air is related to ozone in outdoor air, to ventilation, to reaction with other chemicals (ozone-initiated indoor chemistry) and to the release from furniture and building materials. Chemicals usually reacting with ozone in indoor environments are isoprene, styrene, terpenes, squalene and saturated and unsaturated fatty acids from furniture, carpets, cleaning products and air fresheners. One important source of ozone pollution in indoor air is the release by laser printers, photocopiers, electrostatic filters and precipitators [16].

Exposure to mild concentration of ozone (160 μg/m3) can cause airway irritation, inflammation, obstruction and bronchial hyperresponsiveness. Respiratory symptoms associated with ozone exposure are more frequent in subjects with pre-existent respiratory diseases. In allergic patients ozone could prime the airways enhancing the response to inhaled allergens. Genetic could explain the large variety of short-term reactions to ozone in different subjects. An increase in outdoor ozone concentration has been associated with an increased mortality and hospital admission for respiratory diseases. It has been also shown that outdoor exposure to ozone is correlated with higher morbidity for cardiovascular diseases [17].

Phthalates are a large family of chemical compounds used in the indoor environment to increase the flexibility and elasticity of polyvinylchloride-based carpets. Even if phthalates are mainly introduced in the body by the oral route, respiratory exposure by indoor air has been demonstrated. Phthalates are mainly known as endocrine disruptor, but some study pointed out a possible role in allergy and asthma [18].

Particulate matter (PM) is often classified by its diameter in ultrafine (PM 0.1), with a diameter <0.1 μm; in fine (PM 2.5), with a diameter <2.5 μm; and finally in PM10, with particles <10 μm. Fine particles are mainly produced during combustion and larger particles (PM10) are usually the results of fine-particle aggregation [19]. Major sources of particulate matter in indoor air are combustion process, tobacco smoke, laser printers and photocopiers [15]. Other less prominent sources are pollens, moulds, bacteria and sprays. Usually larger particles are coming mainly from outdoor through windows and the ventilation system. Toxic effects of particulate matter are related to its diameter, and larger particles affect the nose, throat and larger airways, while smaller particles, belonging to the respirable fraction, could directly affect small airways and alveoli. Moreover, the toxicity is also mediated by the substances (e.g. aromatic hydrocarbons, nitrous compounds, aldehydes) absorbed onto the particles’ surface [20]. A large number of studies show that PM could be a strong eye and airway irritant and could exacerbate asthma. Nevertheless, some reports suggested a possible role of particulate matter to induce a direct alveoli inflammation with diffusing capacity impairment [15]. Long-term exposure to PM is able to increase morbidity and mortality for respiratory and cardiovascular disorders. Indoor air threshold for PM has not been established yet as it was done for outdoor exposure to PM.

Exposure to environmental tobacco smoke (also named “passive smoking” or “second-hand tobacco smoke”) is the inhalation of tobacco smoke by non-smoking subjects because of the presence of smokers in their same environment. ETS is one of the most import pollutants of indoor working environment [21]. Tobacco smoke accounts for more than 4000 toxic substances with carcinogenic, toxic and inflammatory effects. Health effects of tobacco smoke on airways are irritation and inflammation, causing asthma and chronic bronchitis exacerbations and increased susceptibility to respiratory infections. Long-term exposure is correlated with airway impairment and chronic obstructive pulmonary disease. ETS has been classified carcinogenic by the IARC, increasing the risk of lung cancer by 16–19 % in occupational settings. Some reports suggest an increased risk also for breast cancer [22]. Recent smoking bans in different countries have reduced a lot the exposure to ETS. In indoor environment it has been described also the third-hand smoke phenomenon, when after ETS cessation, smoke-related chemicals can persist in the indoor environment absorbed onto the PM [23].