## 1 Introduction  
Preventure conservation in exhibition halls is a complex challenge frequently conditioned by items exhibited, visitors' regimes, and the near environmental characteristics. Reference literature has usually dealt with issues concerning environmental protection of collections [1] as well as conservation concepts for curators and natural history collections [2; 3]. The effects of pollutants in the museum environment (storerooms and exhibition halls) have been studied [4; 5], and guidelines on preventive conservation have also been published [6]. Very often, environmental conditions needed by goods do not fit well with those more comfortable for visitors, even though items exhibited are located inside showcases or in some kind of protected places. Moreover, outdoor environment, air quality, and potential pollution risk from urban locations could also affect long-term conservation [7; 8]. Indoor air pollution and its control has been analyzed to design adequate conservation strategies [9], and microclimates generated by pollutants in museums have been monitored to propose preventive conservation measures [10]. In addition, the use of museum spaces as places for socialization and interaction between the general public and the museum culture makes the museum environment a fragile system that needs to be stabilized for proper preventive conservation of exhibited items [11; 12]. Accordingly, the evaluation of the museum air quality and the limit of pollutants has been also investigated [13]. In fact, these concepts and starting assumptions are the same or very close to those considered for storerooms of museums where many items, which are even more abundant than those exhibited, must be properly conserved [14]. Environmental monitoring by usingsensors has proved to be a useful strategy for protecting cultural and natural items, especially those sensitive to pollutants and susceptible to deterioration or corrosion [15]. Within this subject, previous research was conducted in the Museo Nacional de Ciencias Naturales (MNCN) using environmental pH sensors. Resulting data pointed out the meaning and relevance of the deviation from neutral conditions and how it should be considered a direct methodology to evaluate air quality and real preventive conservation conditions. Environmental pH sensors are based on sol-gel technology; they consist of a thin coating applied upon a common soda lime glass slide. The coating matrix is composed of a polysiloxane network obtained from a silicon alkoxide in which a sensitive organic dye, against pH, is immobilized into the porous coating structure. Partial thermal densification of the whole coating allows one to obtain a material able to exhibit sensitivity to air chemical acid/base species (due to its residual porosity) and is stable against water, chemicals and handling [16]. Some former investigations carried out in different museums demonstrated that these sol-gel sensors are useful to indicate the current status of environmental pH in exhibition halls, showcases, storerooms, etc. They are also informative about pH variations due to natural or controlled ventilation, the impact of an unusual number of visitors, or when a special event took place in the museum spaces or in its surroundings [17; 18]. Therefore, environmental pH monitoring by using these sensors is a useful tool to check preventive conservation conditions quickly, since no more than 24 h is needed for sensitization of sensors with accuracy high enough to detect small variations on air quality (sensor accuracy is estimated to be \(\pm 0.1\) units of the pH scale) [19]. Other properties and characteristics of pH sensors are reversibility, reuse, low cost, and miniaturization possibility, or adaptation for every real need in museum spaces. In addition, sensors can be used to check outdoor pH, which allows an air quality comparison indoors and outdoors, as well as an assessment of the impact of potential urban pollution on museum collections. One of the most practical properties of sensors is reversibility.