# The Interplay between Air Quality and Energy Efficiency in Museums, a Review
## 3 Results and Discussion
### IAQ in Museums  
It is well established that air pollutants can harm works of art, and in some cases the damage is irreversible [33; 57]. For example, O\({}_{3}\) and NO\({}_{2}\) concentrations must be close to zero in order to preserve works of art for an extended period of time [8; 58]. Particulate matter in the air comes from both indoor and outdoor sources. The impact on works of art is determined by their size, concentration, and chemical composition [52; 59]. The concentration and chemical composition of airborne particulate matter inside and outside of five museums in California (USA) were studied as part of an extensive monitoring program [60]. The Sepulveda House is an historical building with natural airflow and this museum had the highest indoor/outdoor (I/O) fine PM ratio = 0.94, i.e., the same levels as outdoors. On the contrary the Norton Simon Museum (with HVAC and PM filtering system) had the lowest I/O fine PM ratio, which was 0.18. The concentrations of black elemental carbon particles and fine soil-dust particles in all of the museums ranged from 20 to 100% of the outdoor levels, depending on the presence or absence of an HVAC and PM filtering system. At the sites with a HVAC and a PM filtering system, the indoor airborne particles were mostly fine, consisted largely of organic matter and at two sites, indoor concentrations of organic matter exceeded the respective outdoor concentrations. According to the authors, PM deposition can affect the surfaces of works of art by soiling them or by attacking them chemically, depending on PM chemical composition [60]. Table 1 compares guidelines developed by the World Health Organization (WHO) for the most common air pollutant concentrations for human health protection, with guidelines proposed for the preservation of housed works of art for the same pollutant. The preservation target is the amount of time (in years) that the objects can be exposed to the indicated level of pollutants without deterioration. Table 2 summarizes the findings of some monitoring campaigns that measured indoor and outdoor pollutant concentrations (and the respective I/O ratio) in museums or historical buildings. Only the results for the pollutants presented in Table 1 are included in Table 2. It should be noted that the average time for the reported atmospheric pollutant concentrations and the applied monitoring method varies by museum. \begin{table}
\begin{tabular}{c c c c c} \hline  & & **Human Health** & **Museums** & **Museums** \\  & & **WHO (2021)**[61] & **[PERSON] (2003)**[9] & **[PERSON] (2006)**[57] \\ \hline Main sources & Air pollutant & Indoors or outdoors &
\begin{tabular}{c} Maximum concentrations \\ for one year preservation \\ \end{tabular} & Sensitive materials & Other materials \\ Outdoor & O\({}_{3}\) (ppb) & 50 (24-h) & 5 & \textless{}0.05 & 0.5–5 \\ Outdoor & NO\({}_{2}\) (ppb) & 5 (annual) & 5 & \textless{}0.05–2.6 & 2–10 \\ Indoor & PM\({}_{2.5}\) (\(\mu\)g m\({}^{-3}\)) & 5 (annual) & 10 & \\ \hline \end{tabular}
\end{table} 
Table 1: Proposed guideline values for air pollutant concentrations. \begin{table}
\begin{tabular}{l c c c c c c c c c c c} \hline \hline  
\begin{table}
\begin{tabular}{l c c c c c c c c c c} \hline \hline
**Museum/Mistorial Building** & **O\({}_{\text{d}}\)** & **O\({}_{\text{d}}\)** & **O\({}_{\text{d}}\)** & **NO\({}_{\text{d}}\)** & **NO\({}_{\text{d}}\)** & **NO\({}_{\text{d}}\)** & **Fine** & **Fine FM (Oo)** & **PM (Oo)** & **PM (Oo)** & **References** \\
**Museum/Mistorial Building** & **(Mai)** & **(Guo)** & **(Io)** & **(Io)** & **(Io)** & **(Io)** & **(Io)** & **(Io)** & **(Io)** & **(Io)** & **(Io)** \\
Several studies, both old and new, have reported particle number concentrations and size distribution, PM chemical composition, VOCs, organic and inorganic acids, SO\({}_{2}\), H\({}_{2}\)S, CO\({}_{2}\), and microorganisms [16; 17; 28; 32; 72; 73; 74; 75; 76; 77; 78; 79; 80]. Table 2 shows a small sample of such studies to demonstrate that the issue of elevated indoor air pollutants is always current.