The statistical analysis revealed that out of the 420 individuals who participated from 407 dwelling units, 25% were male and 75% were female, with an average age of 33 years. 32 individuals were smokers (25 females and seven males), six individuals reported respiratory or dermal problems, and 39 individuals spent more than 10 h per day outdoor. Smokers, individuals with respiratory or dermal problems, and those who spent more than 10 h engaged in activities outdoors were considered as ineligible participants. Consequently, after filtering out the ineligible individuals, the questionnaires that were completed by 301 females and 57 males were considered eligible for further evaluation. The characteristics of the eligible participants are presented in Table 2. The youngest participant was a 16-year-old female, while the oldest was a 77-year-old man.

Out of 358 eligible individuals, approximately 38.2% of participants (comprising 117 females and 20 males) indicated that they had experienced at least one of the general, mucosal, or dermal symptoms weekly over the past three months, with no identifiable cause. The most frequently reported symptoms were general symptoms, noted by 28% of individuals. Dermal and mucosal symptoms followed as the second and third most common, reported by 15% and 10% of participants, respectively. The prominence of general symptoms aligns with findings from other studies conducted in residential environments [8, 42]. The prevalence of SBS symptoms was observed at 39% in females and 35% in males. Many studies reported higher prevalence of SBS in females than in males [12, 34], due to the difference in the perception, social roles, personality traits and physiological characteristics [5]. The share of females among the eligible participants is 82% and this imbalance may have an impact on the result. Nonetheless, the statistical analysis in this study revealed no significant association between gender and the prevalence of SBS symptoms (P-value = 0.59). The prevalence of each specific symptom related to sick building syndrome is detailed in Table 3.

Based on the data in Table 3, the most commonly reported symptom was fatigue, with a prevalence of 17.04%. Following that, concentration difficulty, headache, feeling of heaviness in the head, dry facial skin were the second to fifth most common symptoms, with prevalence rates of 12.01%, 11.45%, 10.06%, and 9.22%, respectively. The least reported symptom was nausea, with a prevalence of 1.12%.

According to the questionnaire results, noise was identified as the most significant irritating factor on a weekly basis by 22.91% of the study participants. Following this, the presence of dust and dirt and passive smoking were ranked as the second and third most significant factors, respectively, with a considerable margin between them and noise. These findings are presented in Table 4.

The statistical analysis assessing the impact of environmental factors on the prevalence of SBS symptoms identified significant associations between SBS prevalence and dim light (P-value = 0.012, OR = 2.1, CI = 1.09–4). Additionally, a notable correlation was found between SBS and noise pollution (P-value = 0.031, OR = 1.75, CI = 1.1–2.9), passive smoking (P-value < 0.01, OR = 2.6, CI = 1.22–5.4), static electricity (P-value < 0.01, OR = 3.8, CI = 1.15–12.6), bad air (P-value < 0.01, OR = 4.6, CI = 1.6–13), and too high room temperature (P-value = 0.039, OR = 2.6, CI = 1.13–5.95), with the significance level of α = 0.05. However, as stated in Table 4, no significant associations were found between SBS and other independent parameters at a significance level of α = 0.05. Besides, the association between four most prevalent symptoms and the affecting risk factors has been investigated via logistic regression in MedCalc statistical software. The association between fatigue, headache, heavy head and the concentration difficulty and 14 risk factors are expressed as Odds ratios in Fig. 3.

Association between weekly prevalence of a) fatigue, b heavy head, c headache and d) concentration difficulty and 14 risk factors

Figure 3 reveals that there is a positive association between static electricity and fatigue, where a negative association is observed between varying room temperature and fatigue. Heavy head was significantly associated with passive smoking (Fig. 3.b). In addition, passive smoking has a positive correlation with headache and concentration difficulties, where the association is more significant for headache (Fig. 3.c and d).

It was observed that all participants experiencing weekly static electricity were women (P-value = 0.002), likely due to indoor slippers that prevent static electricity dissipation. Moreover, when examining the sensation of high room temperature, a significant association was noted between feeling high temperature and SBS symptoms in women (P-value = 0.025). However, no significant association was found among men (P-value = 0.555). One plausible explanation for this observation is the clothing style of women in Iran, which is influenced to some extent by Islamic culture. Older women, in particular, tend not to wear comfortable clothing indoors, leading to a perception of high temperatures among women. Segmenting females into three age groups (under 30, 30–40, and over 40 years) demonstrated a significant relationship between age and the perception of high indoor temperature (P-value = 0.016).

Given that traffic-induced noise pollution is a significant environmental concern in Rasht [28], the influence of noise as a factor affecting SBS is anticipated. Passive smoking and poor air quality (bad air) in residential units may also be attributed to inadequate ventilation. Moreover, the sensation of too high room temperature can be caused by a lack of proper ventilation in residential units or improper energy management in buildings.

Rasht is geographically divided into five urban districts (Fig. 1). Typically, residents in district one, located in the northern areas of the city, exhibit higher financial security compared to other districts, while residents in district five, located in the southern areas of the city, possess relatively lower financial security. Based on the questionnaire statistics, 27%, 20%, 12%, 31%, and 9% of the participants are residents of districts one to five, respectively. Upon analysis of the questionnaire results, it was found that 47% of the participants in district one reported experiencing at least one SBS symptom on a weekly basis. The corresponding percentages for districts two to five were identified as 42%, 26%, 32%, and 37% (Table 3). Among the environmental factors that significantly impacted SBS, only environmental noise exhibited variation across different urban districts, displaying a relatively distinct pattern. Consequently, the relationship between noise and SBS symptoms was separately examined in the five districts, with the results presented in Table 5.

The findings revealed the strongest association between SBS symptoms and noise in district four (OR = 4.13, CI = 1.7–10.2). Furthermore, district one (OR = 2.03) and five (OR = 1.5) exhibited significant associations. These outcomes are consistent with the fact that a major east-west transportation route of the province runs through district four. Additionally, district one experiences high traffic volumes due to the presence of numerous healthcare facilities, which subsequently leads to a significant impact of noise on the prevalence of SBS symptoms. Although the traffic noise is not an indoor factor, the improper acoustic design of buildings can enhance the noise impact indoors. Besides, the lack of considering noise map in residential land use planning as well as incompatibility of building acoustic with the background SPL increase the potential of SBS and BRI in Rasht city. Prolonged exposure to environmental noise can transform nonspecific symptoms into lasting physical, cardiovascular, and psychological BRIs [35].

To investigate the risk factors contributing to SBS in residential units in Rasht, a field study were conducted in 45 residential units, with results presented in ESM_5. The situation of six risk factors measured in the studied units are depicted in Fig. 4. The noise measurements in the living rooms of these units indicated that the average SPL over a 30-minute period ranged from 53 to 68 dBA. This exceeded the Iranian daytime standard limit of 55 dBA [28] in 75.5% of the examined residential units. It is signifying that the environmental noise level in residential units in Rasht is significantly high and is a major contributing factor to symptoms related to SBS, as also evident in the questionnaire results presented in the previous Sect. “Questionnaire results assessment”.

The distribution of observed risk factors in 45 units

As stated in Sect. “Questionnaire results assessment”, inadequate ventilation is another major factor contributing to SBS in Rasht. The ventilation rate of the living rooms was measured using the direct measurement of CO2 concentration and calculated using Eqs. (1) and (2). The calculation results showed that the average ventilation rate in the examined units was 20 L per second per person, ranging from a minimum of 2.7 to a maximum of 63.5 lit/(p.sec). Nonetheless, 65% of the residential units had ventilation rates below 20 lit/(p.sec). According to Fig. 4, about 20% of the studied units had ventilation rates below the national code for the minimum amount of fresh air intake in residential units [11] (7.1 lit/(p.sec)), indicating inadequate ventilation directly affects various issues in residential units such as passive smoking, lack of access to fresh air, and increased room temperature.

Furthermore, dim light in residential units has been identified as a primary contributor to SBS according to the questionnaire survey results in Sect. “Questionnaire results assessment”. It was observed that 32% of 45 units had either low or moderate lighting in the living rooms (Fig. 4), whereas the remaining residential units had adequate natural daylighting. In this regard, Siu-Yu Lau et al. [41] reported that 90% of respondents were satisfied with the luminous condition in living rooms of Hong Kong. Also, a research by Xue et al. [49] revealed that 54% of participants in Hong Kong reported luminous comfort in the living room, from which 60% satisfied with the daylight quality.

The residential units in Rasht and northern Iran are generally designed with larger windows in the living rooms than in the bedrooms, which leads to a reduced amount of natural light entering the bedrooms. The results revealed that satisfaction levels regarding natural light in residential units in Rasht were less than 70% (Fig. 4). However, natural light reception can also be influenced by factors such as weak sunlight during autumn and winter seasons, cloudy and rainy weather, solar altitude angle, building orientation, floor, and sociocultural influences, which were not considered in this research and need further investigation.

The houses surveyed in this study had an average age of 14 years, ranging from 3 to 40 years. The statistical analysis revealed no significant relationship between the age of residential units and SBS symptoms at the alpha level of 0.05 (P-value = 0.66). The heating systems present in the units consisted of both central radiator heating and individual natural gas heaters, with 62% of the units employing central heating, while the remaining units utilized individual natural gas heaters (Fig. 4). There was no significant association observed between the type of heating system and SBS symptoms (P-value = 0.10).

Regarding interior walls, 38% of the units had painted interior walls, 42% were covered with gypsum plaster, and 20% utilized wallpaper (Fig. 4). The statistical results indicated no significant relationship between SBS symptoms and the use of paint for interior wall finishing, both overall and under different ventilation conditions, at the alpha level of 0.05 (P-value = 0.6).