The quality of household environment is rapidly gaining importance as a public health issue worldwide, especially regarding respiratory diseases and allergic disorders. In fact, the specific climatic characteristics, the reduced ventilation, the presence of identified indoor allergens such as mold, dust mites, animal dander and cockroaches, along with the ambient concentrations of pollutants from the combustion of domestic devices and tobacco, make indoor environment a mixture of risk factors for the development of allergies . A large number of studies have been performed in the last decades dealing with the indoor air pollution, and most of them aimed to identify its possible association with health outcomes in children. In the case of the present study, we aimed to identify the potential association between the presence of AD in 6-to-7-years old population and the use of electric, gas and biomass heating and/or cooker at home. Unfortunately, our results failed to show a clear relationship between them, which will be discussed in next lines.
Although indoor air pollution is a huge public health concern, little research has been conducted on the relationship between domestic fuels and AD [6, 12]. Respiratory disorders, asthma and rhinitis are the main studied outcomes because the inhalation of pollutants is considered to be the main source of exposure. However, it should be taken into account that the skin is the first human body’s main line of defence against external factors, and if altered, its functional barrier would be deteriorated facilitating the entrance of chemical and biological elements into the internal compartments . Besides that, most of the evidences are based on studies that have been carried out in developing countries where living conditions differ from those in Spain [20, 21]. As it is said above, our study population belongs to a developed country where the technology advances have extensively released new gas modern heating appliances. These are provided with closed conductions for residual gases of the combustion process, more efficient and insulated devices and cleaner installations . So, they are expected to be safer in terms of toxicological risk factors comparing to this kind of heaters used in developing countries.
On the other hand, the strongest evidences are those that link the use of biomass domestic devices and the health outcomes in childhood [4, 20, 22]. The World Health Organization stated that in 2002, Sub-Saharan Africa and South-East Asia led with 396 000 and 483 000 deaths due to indoor smoke, respectively [23, 24]. It is established that the combustion of biomass fuels releases more polluting substances to indoor environments than gas burners, so its potential toxicity is higher too [20, 24]. Fortunately, in Spain, biomass is much less frequently used than either gas or electricity as a domestic fuel. In fact, the use of biomass fuels is associated with more rural and poorer areas; specifically such fuels are burned in fireplaces, firewood stoves and cookers, charcoal burners and other simple heating devices, and these are no longer widely used in our country. Actually, in our study population, the number of biomass users was very limited and nobody used biomass as unique energy source at home.
Although the present study did not obtain sufficient evidences to conclude that the use of combustion fuels at home could be associated with the development of AD, the role of electricity as a ‘clean’ domestic energy source is not clear. An almost statistically significant association was found for the use of electric heater despite being considered a clean source of energy in terms of releasing polluting substances into the air . As it is said above, the toxicological hypothesis should be put aside in the present study due to the developed characteristics of the studied population. Otherwise other possible reasons should be considered: firstly, it would be necessary to assess the potential effect of electromagnetic fields generated by electrical appliances on children′s immune systems and their effect on AD development in particular. In fact, the presence of electromagnetic fields has been shown to affect the immune system  and may, therefore, be involved in some way in the aetiology of AD . Secondly, these results might be in line with the “hygiene theory”; it hypothesises that the non-exposure to risk factors during childhood and development of infections results in a lack of exposure to the necessary stimuli to help the immune system mature with the following inception of allergic disorders [2, 27]. And finally, it should be noted that the nature and efficiency of heating systems may influence the development of allergic disorders by modifying the climate indoors . For example, it has been shown that the environmental humidity can alter the mast cells increasing the histamine content in the dermis, which results in the modification of the allergic process . In this case, it is known that the use of electric heaters, including air conditioning systems, which also act as emitters of hot air, may contribute to dry the indoor environment. By the same token, changing indoor climate due to the use of heaters at home could modify the effect of other allergens [30, 31].
As expected, the present work showed that the mother’s level of education and the geographic area influence the distribution of the percentages of use of each domestic device. Likewise, it seems that the choice of a specific fuel did not follow the same pattern for cookers and heating appliances. For instance, it is shown that users of electric cooker did not use the same energy source for heating purposes, as it could be observed with the low number of subjects included in the ‘nature of energy source’ variable. This could be a reason why it was difficult to obtain the ‘pure’ contribution of each fuel to the development of AD.
Although the ISAAC questionnaire has been validated and it has been widely used in epidemiological studies involving allergy in children, the presence of AD or not in individuals is based on parents’ self-reports through two questions. This evidently shows the typical limitations in verifying the diagnoses of AD . Also, the ISAAC phase III in Spain did not collect information about parental history of allergy. Likewise, it should be taken into account that the results regarding the environmental risk factors could be influenced by the presence of reporting bias, as all self-reported exposure questionnaires show.
In order to assess the real role of domestic fuels used at home in AD development, other variables should be considered: whether there are fume extraction systems and, if so, their characteristics, the home size, each cooking/heating system type, fuel quality, etc. The amount of time a child spends at home, especially near combustion systems, should also be assessed. It is also important to remember that despite the general belief that the more ventilation, the lower exposure to indoor pollution, it also implies more exposure to outdoor air; that is, external environmental pollutants may also enter homes. This means that, in practice, it is very difficult to determine children’s exact level of exposure to the various fuels or residues released at home by indoor systems. On the other hand, due to lacking data, this study has not attempted to assess the degree of exposure to fuels in closed places other than their home where children spend much of their time (i.e., schools), the presence of other allergens in domestic environments (i.e., mould, dust, mites, etc.) [32, 33], dietary habits  or alternative variables which represent in a more accurate way the social status of the family. Accordingly, it should be taken into account the possible presence of residual confounding in the present analysis.