## 2 Methodology
### Method of Human Health Risk Assessment  
This study assessed the health risks of IVOCs based on observational data from published papers. The assessment used the inhalation exposure-based method recommended by the Risk Assessment Guidance for Superfund Volume I: Human Health Evaluation Manual (Part F, Supplemental Guidance for Inhalation Risk Assessment) (EPA-540-R-070-002) from the U.S. Environmental Protection Agency (EPA) and the Technical Specifications for Health Risk Assessment of Ambient Air Pollution (WS/T 666-2019) published by the Chinese National Health Commission [24; 25]. It assessed the carcinogenic and non-carcinogenic risks of selected IVOC species, so as to comprehensively evaluate the potential impacts of IVOCs on human health via the inhalation route. The relevant parameters were from the Integrated Risk Information System (IRIS, [[https://www.epa.gov/iris](https://www.epa.gov/iris)]([https://www.epa.gov/iris](https://www.epa.gov/iris)), accessed on 6 April 2025) and the Exposure Factors Handbook of the Chinese Population: Adults. Based on the IRIS, this article will first identify the IVOCs that pose health risks to humans from the existing research, prioritize them in regard to risk assessments, and compile toxicity data for these compounds. Second, it will present exposure data and estimate exposure levels. Finally, using ambient IVOC concentrations and exposure information, this study will detail assessments conducted on both the carcinogenic and non-carcinogenic risks. For the carcinogenic risk assessment of IVOCs, the Excess Cancer Risk (ECR) was generally used for the quantitative analysis. The ECR was characterized by the inhalation unit risk (IUR), which is the lifetime cancer risk per microgram, per cubic meter (\(\upmu\)g m\({}^{-3}\)) of atmospheric pollutants. The ECR was calculated as per Equation (1):  
\[\text{ECR}=\text{IUR}\times\text{EC} \tag{1}\]  
When the ECR \(\leq 10^{-6}\), the cancer risk is negligible; when \(10^{-6}<\text{ECR}\leq 10^{-5}\), there is a moderate cancer risk; when \(10^{-5}<\text{ECR}\leq 10^{-4}\), there is a high cancer risk; and when ECR \(>1\times 10^{-4}\), there is a very high carcinogenic risk. Figure 1: The proportion of different types of papers collected. The grass-colored green section in the figure represents field observation papers. The arcs represent the distribution of observations from different sites in the field observation studies. For the non-carcinogenic risk assessment of IVOCs, the Hazard Quotient (HQ, dimensionless) was used for the quantitative analysis, which is the ratio of the exposure concentration to the toxicity value. The HQ was calculated as per Equation (2):  
\[\text{HQ}=\text{EC}/\left(\text{RFC}\times 1000\right) \tag{2}\]  
When HQ \(\leq\) 1, there is a low non-carcinogenic risk; when 1 \(<\) HQ \(\leq\) 2, there is a moderate non-carcinogenic risk; when 2 \(<\) HQ \(\leq\) 3, there is a high non-carcinogenic risk; and when HQ \(>\) 3, there is a very high non-carcinogenic risk. The exposure concentration (EC) was calculated as per Equation (3):  
\[\text{EC}=\text{CA}\times\text{ET}\times\text{EF}\times\text{ED}/\text{AT} \tag{3}\]  
The meanings of the parameters in the equations are shown in Table 1.  
Health risk models inherently involve uncertainties. For example, exposure parameters may vary due to differences in the population characteristics, such as age, gender, and sensitivity. Exposure concentrations can also be uncertain due to factors like instrument precision, model accuracy, and variations in human activity patterns. Additionally, the toxicity values of pollutants often involve uncertainties because they are typically extrapolated from animal studies to humans. However, the technical specifications for health risk assessments of ambient air pollution do not provide error ranges [25], and it is difficult to calculate these uncertainties. As a result, it is difficult to clarify the margins of error in the model.