## 4 Discussion
### Human Exposure to Benzene  
Humans exposed to benzene can present serious negative effects. For instance, benzene exposure reduces the abundance of immune cells in multiple immune organs, such as bone marrow, thymus, spleen, and liver [22]. Furthermore, exposure during in utero fetal development can cause fetal abnormalities [23]. Therefore, exposure to benzene, in both private and public environments, should be investigated in-depth. In the present study, 8 out of the 10 incense sticks tested resulted in the emission of benzene exceeding 17 ug m\({}^{-3}\), which is the guideline value presented by the World Health Organization (WHO) [1] (Figure 2). Figure 1: (**a**) Graph of the correlation between the length (cm) of the combustible part of the incense stick and the emitted concentration of benzene (\(\mu\)g m\({}^{-3}\)); (**b**) graph of the correlation between the diameter (cm) of the combustible part of the incense stick and the emitted concentration of benzene (\(\mu\)g m\({}^{-3}\)). The European Union currently abides by WHO guidelines for air quality, which are centered on assessing health and hygiene implications. These guidelines utilize an evaluation of acceptable risk levels in consideration of all relevant scientific evidence and establish concentration thresholds below which lifetime exposure (or exposure for a specific time frame) is considered safe for human health. Precisely, the guideline by WHO for carcinogenic substances established a concentration of 17 \(\upmu\)g m\({}^{-3}\) for exposures throughout the life span and throughout the working life, without suffering harmful health effects [24]. In Italy, as well, no specific regulation on residential indoor air quality is currently in force [25; 26; 27]. On the contrary, in Italy, levels of workplace exposure to carcinogens and mutagenic agents have been established in Legislative Decree No. 81/2008, which aims at the protection of health and safety at work. TLV-TWA, established by Legislative Decree No. 81/2008, is 3.25 \(\upmu\)g m\({}^{-3}\). However, Legislative Decree No. 81/2008 is not applicable to residential environments [28]. Furthermore, since 1938, the American Association of Industrial Hygienists (ACGIH) has focused on health in the workplace. Annually, the ACGIH publishes the Threshold Limit Value (TLV) for workplace exposure to benzene (1.6 mg m\({}^{-3}\) for an 8-h exposure). The relative non-occupational indoor exposure is ((TLV-TWA)/10), namely, 1.6/10 is equal to 160 \(\upmu\)g m\({}^{-3}\). The values determined in the present study, using different varieties of incense, are 2.5 times lower (considering the upper extreme value of 66.5 \(\upmu\)g m\({}^{-3}\), recorded by stick #7 variety). The key factor in ensuring proper exposure assessment is to consider the environment in question, the methodology for incense stick use, and the relevant exposure references, as the reference limit for exposure is identified for an 8 h workday, while the value discovered pertains to a single incense stick's combustion (approximately 40 min) and may require normalization. Furthermore, over the last couple of decades, the European Union has importantly focused on air quality in indoor environments. For instance, the European project INDEX (Critical Appraisal of the Setting and Implementation of Indoor Exposure Limits in the EU) has aimed to assess indoor limit values [29; 30]. The study identified formaldehyde, nitrogen dioxide, carbon monoxide, benzene, and naphthalene as priority pollutants for European regulation, suggesting exposure limits, recommendations, and control measures to minimize health risks. The publication of those guidelines ended the use of risk assessments for Occupational Exposure Limit Values (OELVs) or TLVs proposed by the ACGIH, which were reduced by 1/10 or 1/100, respectively. It is notable that the World Health Organization reaffirmed this guidance in the early 1980s through the publication of \"Indoor Air Pollutants: Exposure and Health Effects\".