Introduction

Air pollution is a silent killer of human health and has become a great public concern in past decades (Cohen et al. 2017; Miller 2020). Since the promulgation of the air pollution prevention and control plan has come into act in China from 2013, other air pollutants have significantly improved, but the ozone (O3) level has remained consistently high (Han et al. 2020; Wang et al. 2022). Recently growing evidence suggests that exposure to O3 in association with adverse health effects has extended to multiple organ systems (Theis et al. 2014), such as the respiratory system (Gu et al. 2020) and cardiovascular system (Kazemiparkouhi et al. 2020), but its harmful effect on liver is not neglected and has attracted more attention for research (Alewel et al. 2021). Thus, it is an urgent need to identify the effect of O3 exposure on early biomarkers of liver function, which may give us a chance to take effective action to prevent the occurrence of chronic liver disease.

Non-invasive markers may be valuable and costless to identify individuals who are more likely to develop severe liver events, which may have high ability to predict physician-diagnosed severe liver-related outcomes (Lee et al. 2021). EASL-EASD-EASO clinical practice guidelines have recommended that fibrosis-4 (FIB-4), aspartic aminotransferase (AST)/ platelet count (PLT) ratio index (APRI), and other non-invasive biomarkers can be used to predict fibrosis staging and liver-related events (EASL. et al. 2016; Siddiqui et al. 2019). Liver fibrosis is characterized by abnormal proliferation of connective tissue in the liver (Nah et al. 2021; Toosi 2015), and it is rarely diagnosed before cirrhosis because it is asymptomatic in human. It is well known that liver fibrosis plays a vital role in developing cirrhosis and hepatocellular carcinoma (Watt et al. 2019), and population-based studies have furthermore indicated that hepatic fibrosis is an important predictor of liver-related mortality and all-cause mortality (Long et al. 2021). However, studies of the effects of air pollution on liver damage have been inadequate (Guo et al. 2022; Hou et al. 2022; Pejhan et al. 2019). Since air polltants can interact and permeate with each other to maintain dynamic balance, people are inevitably exposed to indoor and outdoor air pollution in daily life (Chafe et al. 2014). About one-third of indoor air pollution (IAP) was derived from the uncompleted combustion of domestic solid fuel (Li et al. 2017), and it might contribute to increased outdoor air pollution level (Yun et al. 2020). IAP is related to increased risk of chronic diseases such as cancer and chronic liver disease (Yusuf et al. 2020). Moreover, animal studies have found that O3 or fine particulate matter (PM2.5) may cause metabolic disorders, e.g., glucose and lipid metabolism, as well as expression of specific proteins in rats by inducing the same mechanisms of oxidative stress and inflammation in liver cells (Miller 2020; Theis et al. 2014; Xu et al. 2019). It is noted that harmful substances inhaled in air are metabolized in the liver and induce liver injury because the liver is the major detoxifying organ in the body (Ya et al. 2018). The association of O3 exposure with liver dysfunction is little investigated and its interaction with IAP on liver dysfunction is urgently explored, which may provide more accuracy to assess air pollution-related liver dysfunction.

In this paper, we examined a large-scale health survey data of rural adults in China to explore the associations of ambient O3 exposure, indoor cooking fuel type use, and their possible interaction effects with hepatic fibrosis indices (FIB-4, APRI, and AST/alanine aminotransferase (ALT)). The findings of this study may provide some clues to identify the O3-related liver dysfunction and provide the corresponding effective control measures.

Method

Study population

The study participants were derived from the baseline survey of the Henan Rural Cohort Study, which was carried out between July 2015 and September 2017 in Henan province, China. In brief, a questionnaire was used to collect personal information, socioeconomic status, daily lifestyle, physical health status, etc., and the results of physical examination were recorded for each participant, including anthropometric measurements (such as height, weight, blood pressure, and body fat rate) and blood biochemical test (such as fasting blood glucose, cholesterol, AST, and ALT), and detailed introduction of study design and data collection was described elsewhere (Liu et al. 2019). In this study, after excluding participants with viral hepatitis or without measurement of hepatic fibrosis indices (n=9806), and participants without cooking (n=8378) or partially missing cooking-related information (n=65), a total of 21,010 eligible cooking participants were ultimately included. The characteristics of excluded and included study population are shown in supplementary materials (Table S1). Ethical approval from the Zhengzhou University Life Science Ethics Committee (Code: [2015] MEC (S128)) was received, and the written informed consent from each participant was obtained prior to this study.

Covariates

The covariates included in the analysis are age (continuous), gender (male or female), educational levels (elementary school or below, middle school, and high school or above), marital status (married/cohabitating and unmarried/divorced/widowed), average monthly income (<500RMB, 500-999RMB, and ≥1000RMB), smoking status (never, former, and current smokers), drinking status (never, former, and current drinkers), body mass index (BMI, continuous), physical activity (low, middle, and high) (Tu et al. 2019), and chronic diseases status (type 2 diabetes mellitus (T2DM), coronary heart disease (CHD), and stroke). The diagnostic criteria of T2DM are as follows: participants had fasting blood glucose ≥7.0 mmol/L, or self-reported physician-diagnosed of T2DM and treated with hypoglycemic medications at the same time (Kang et al. 2022). CHD was defined by the self-reported physician diagnosed of CHD (Wang et al. 2019). The information regarding meat fat intake, fruit, and vegetable consumption in the past year was collected by the frequency questionnaire (grams and frequency), which had been confirmed to be suitable for rural adults (Xue et al. 2020b).

Assessment of exposure

The information regarding cooking fuel type was collected by a questionnaire which has been validated in the previous study (Pan et al. 2021). Participants were classified into clean fuel use (natural gas, gas, and electricity) and solid fuel use group (coal and wood) two groups, according to the primary fuel type used for household cooking.

The O3 exposure concentration of each participant was obtained from Tracking Air Pollution in China data set in China (Xue et al. 2020a), which integrated O3 observation data, satellite remote sensing O3 vertical profile, community multi-scale air quality (CMAQ) simulation, weather research and forecasting (WRF) simulation, vegetation index, night light, and population data by using multiple methods including random forest model, elastic net regression model, and the spatiotemporal variance-covariance function. And the spatial resolution of the WRF-CMAQ model is 36 km × 36 km (Xiao et al. 2022), which was verified by using the daily 8-h average surface O3 monitoring data from 1666 stations of the China Environmental Monitoring Center and showed a good prediction ability which indicated by the R2 (R2=70%) and root mean squared errors (RMSE: 29%) of 5-fold cross-validation. Ground-level O3 data for the 3 years prior to the survey date were obtained according to the longitude and latitude of the participant’s residential location, and the average value was calculated and matched as the O3 exposure level of the participant. The O3 exposure was classified into low (<93.07μg/m3) or high (≥ 93.07μg/m3) level by results from restricted cubic spline for subsequent analysis (Fig. S1).

Definition of hepatic fibrosis indices

After at least 8 h of fasting, peripheral venous blood samples were drawn from each participant by a trained nurse using a disposable syringe and stored in a dimethylamine tetraacetic acid tube. Blood routine such as PLT and blood biochemical indexes such as AST and alanine aminotransferase (Dai et al. 2021) were simultaneously detected on the same day by using the XS-500i (SYSMEX, Japan) and Cobas c501 (Roche, Switzerland), respectively. A total of 10% of the samples were measured twice with a coefficient of variation of less than 10% (Yusuf et al. 2020).

Hepatic fibrosis indices were evaluated by using the following formula and classified into low and intermediate-high advanced fibrosis based on previous studies’ reported cutoff values: FIB-4 score = (age (year) × AST (U/L)) / [PLT (109/L) × ALT1/2(U/L)], the cutoff points of low and intermediate-high advanced fibrosis for participants aged <65 years old was 1.3, and ≥65 years old was 2.0 (Hou et al. 2022). APRI = [(AST / the upper limit of normal (ULN)) / PLT (109/L)] × 100 (Pan et al. 2021), the cutoff value of APRI identified participants with low and intermediate-high advanced fibrosis was 0.5 (Dai et al. 2021; Xue et al. 2020a). The cutoff value of AST/ALT identified participants with low and intermediate-high advanced fibrosis was 1.5 (Hou et al. 2022).

Statistical analysis

The continuous and categorical variables were expressed as mean ± standard deviation (SD) and number (percentage), respectively. Student’s t-test and chi-squared test were used to compare the differences of continuous or categorical variables between low and intermediate-high advanced fibrosis groups, respectively. Generalized linear models were used to estimate β values or odds ratios (ORs) and corresponding 95% confidence intervals (CIs) of independent or combined associations of cooking fuel type and ambient O3 exposure with hepatic fibrosis indices reflected by FIB-4, APRI, and AST/ALT, and the potential additive and multiplicative interaction of the two exposures on advanced fibrosis. The quantitative interactions were indicated by the relative excess risk (RERI) and the attributable proportion (AP) of risk due to interaction and synergy index (SI). Covariates were adjusted in three models: Model 1 was unadjusted; Model 2 was adjusted for age, marital status, educational level, and average monthly income in the group of men or women, and gender was added to the total population; and Model 3 was an additional adjustment for lifestyles (smoking- and drinking status, physical activity, high-fat diet, vegetable, and fruit intake), BMI, and chronic diseases status including T2DM, CHD, and stroke. All data analyses were performed by using the SPSS Software (version 26.0, IBM-SPSS Inc., Armonk, NY) and R software version 4.2.1. Statistical significance was set as a two-tailed P value less than 0.05.

Result

Characteristics of the study population by hepatic fibrosis status

Table 1 presents the characteristics of participants with low and intermediate-high advanced fibrosis. Participants with intermediate-high advanced fibrosis were more likely to be female, be less educated, be married, never smoke or drink alcohol, have less high-fat diet, more fruit and vegetable intake, a small number of participants with T2DM, CHD, and stroke, and use clean fuel on cooking in the total population. When we observed the characteristics of the population grouped by FIB-4, APRI, and AST/ALT, respectively, differences by FIB-4 in the selected variables were observed between the two groups (all P<0.05). Although there were no differences in the distribution of marital status, high-fat diet, and T2DM between two groups of low and intermediate-high advanced fibrosis identified by APRI, the differences in the distributions of the other selected variables between the two groups were significant (all P<0.05). There were no differences in the distributions of physical exercise, CHD, stroke, and O3 level between two groups of low and intermediate-high advanced fibrosis identified by the AST/ALT ratio values, but the differences in the distributions of the other selected variables were significant (all P<0.05).

Table 1 Basic information on the characteristics of the study participants
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Independent associations of cooking fuel type or ambient O3 exposure with advanced fibrosis assessed by hepatic fibrosis indices

As shown in Fig. 1, the estimated β values for FIB-4, APRI, and AST/ALT in response to solid fuel users versus clean fuel users in all participants were 0.426 (95% CI: 0.372, 0.480), 0.048 (95% CI: 0.031, 0.065), and 0.064 (95% CI: 0.047, 0.081), respectively (Model 1). And the estimated β values for FIB-4, APRI, and AST/ALT in response to solid fuel users versus clean fuel users in women were 0.385 (95% CI: 0.334, 0.436), 0.046 (95% CI: 0.028, 0.064), and 0.038 (95% CI: 0.019, 0.058), respectively (Model 1).

Fig. 1
figure 1

Independent associations of cooking fuel type or ambient O3 exposure with hepatic fibrosis indices. Estimated β values or ORs and 95%CIs in clean fuel, solid fuel, low O3, and high O3 groups were represented by the corresponding black dot (black lines), gray square (gray lines), orange triangle (orange lines), and blue rhombus (blue lines), respectively.

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The estimated ORs of advanced fibrosis assessed by FIB-4, APRI, and AST/ALT in response to solid fuel users versus clean fuel users were 1.680 (95% CI: 1.568, 1.799), 1.518 (95% CI: 1.391, 1.654), and 1.284 (95% CI: 1.192, 1.382), respectively (Model 1). The estimated ORs of advanced fibrosis assessed by FIB-4, APRI, and AST/ALT in response to solid fuel users versus clean fuel users in women were 1.605 (95% CI: 1.485, 1.734), 1.502 (95% CI: 1.363, 1.654), and 1.144 (95% CI: 1.052, 1.242), respectively (Model 1). Similar results were observed in Models 2 and 3, which included more covariables for adjustment. The estimated ORs of advanced fibrosis assessed by FIB-4 and APRI in response to high O3 exposure versus low level of O3 exposure in all participants were 1.429 (95% CI: 1.352, 1.511) and 1.309 (95% CI: 1.216, 1.410) (Model 1). The estimated ORs of advanced fibrosis assessed by FIB-4 and APRI in response to solid fuel users versus clean fuel users in women were 1.393 (95% CI: 1.308, 1.485) and 1.310 (95% CI: 1.206, 1.424) (Model 1). Similar results were observed in Models 2 and 3, which included more covariables for adjustment.

Interactive associations of cooking fuel type and ambient O3 exposure with the risk of hepatic fibrosis

Table 2 presents the additive and multiplicative interaction effects of cooking fuel type and ambient O3 exposure with the risk of hepatic fibrosis. Significant multiplicative interaction effect of cooking fuel type and O3 exposure on the risk of hepatic fibrosis reflected by FIB-4 in women was found (P interaction=0.04). Significant additive interaction of cooking fuel type and O3 exposure on the risk of advanced fibrosis reflected by FIB-4 in women was also found, and the matched figures of RERI was 0.265 (95% CI: 0.052, 0.477), indicating that relative excess risks were attributed to the additive synergistic effect of solid fuel use and high O3 exposure on advanced fibrosis risk; of AP was 0.170 (95% CI: 0.045, 0.295), representing that 17.0% additional risk of advanced fibrosis risk may be attributable to the synergistic effect of solid fuel use and high O3 exposure; of SI was 1.906 (95% CI: 1.058, 3.432), indicating the synergistic effect of solid fuel use and high O3 exposure on advanced fibrosis risk.

Table 2 Interaction of ambient O3 exposure and cooking fuel type with hepatic fibrosis indices
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Combined associations of cooking fuel type and ambient O3 exposure with advanced fibrosis assessed by hepatic fibrosis indices

Figure 2 exhibits the combined associations of cooking fuel type and ambient O3 exposure with hepatic fibrosis indices. Compared to clean fuel user with low O3 exposure, the estimated β values of FIB-4 in solid fuel users with low O3 exposure or solid fuel users with high O3 exposure in women were 0.346 (95% CI: 0.276, 0.415) or 0.496 (95% CI: 0.423, 0.570); the matched figures for APRI were 0.034 (95%CI: 0.010, 0.058) or 0.073 (95% CI: 0.047, 0.098) (Model 1). Similar results were observed for FIB-4 and APRI in Models 2–3. Compared to clean fuel users with low O3 exposure, the estimated ORs of advanced fibrosis reflected by FIB-4 in solid fuel users with low O3 or solid fuel users with high O3 exposure in women were 1.514 (95% CI: 1.361, 1.683) or 2.271 (95% CI: 2.031, 2.541), and the matched figures for advanced fibrosis reflected by APRI were 1.498 (95% CI: 1.306, 1.715) or 1.924 (95% CI: 1.677, 2.203) (Model 1). These results were not substantially changed in Models 2–3.

Fig. 2
figure 2

The combined association of ambient O3 exposure and cooking fuel type with hepatic fibrosis indices. Estimated β values or ORs and 95%CIs in low O3 plus clean fuel, low O3 plus solid fuel, high O3 plus clean fuel, and high O3 plus solid fuel groups were represented by the corresponding black dot (black lines), gray square (gray lines), orange triangle (orange lines), and blue rhombus (blue lines), respectively

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Discussion

In this study, we observed that high levels of O3 exposure and solid fuel use were associated with elevated hepatic indices or the prevalent intermediate-high advanced fibrosis and their synergistic effects on advanced fibrosis. These were more obviously seen in women, indicating that women might be more vulnerable to the effect of air pollution on liver dysfunction. It revealed the impact of indoor and outdoor air pollution on liver damage, as well as the necessity of cleaner fuel switch and the implementation of effective measures to improve air quality to reduce the risk of liver dysfunction.

The fact that solid fuel use was recognized as a source of IAP is well established. Now, the adverse effect of solid fuel used for cooking on the respiratory and circulatory system has been widely recognized (Lee et al. 2020), while the evidence of solid fuel use–related liver dysfunction is lacking. In fact, the liver, as the major detoxifying organ, is more vulnerable to the adverse effects of air pollutants (Pejhan et al. 2019; Zheng et al. 2015).

It is obvious that numerous pollutants emitted from incomplete combustion of solid fuel, such as PM2.5, polycyclic aromatic hydrocarbons (PAHs), and black carbon (BC) (Ambade et al. 2021a; Ambade et al. 2022) can enter the body through the respiratory tract, and they may also enter the body directly through the skin or other routes when the concentration is high enough. Results from four European cohorts of the ESCAPE project (n=174,770), and the Rome longitudinal cohort (n=1,245,397) showed that exposure to high levels of ambient air pollutants such as PM2.5 and NO was associated with an increased risk of liver cirrhosis or liver cancer (Orioli et al. 2020; Pedersen et al. 2017). Results from the National Health and Nutrition Examination Survey showed that PAH exposure was positively associated with the risk of non-alcoholic fatty liver (Hu et al. 2021). An experimental study showed that PM2.5 was associated with metabolic disorders via inducing oxidative stress and inflammation in liver cells (Ding et al. 2019). Moreover, some evidence showed that air quality had improved for regional closures caused by the COVID-19 pandemic, while the main source of PAHs is still the burning of biomass and coal, which suggests the need to promote the use of cleaner fuels to improve air quality (Ambade et al. 2021b; Kurwadkar et al. 2022).

Currently, epidemiological evidence for ambient O3 exposure–related liver dysfunction is limited. Only one population-based longitudinal study of elderly people showed that short-term (n=545) or long-term (n= 318,911) exposure to O3 was related to a 3.6% increase in AST, and a 4.0% increase in ALT, respectively (Kim et al. 2015; Li et al. 2022). AST and ALT, as the typical liver enzyme in blood samples, are often used to reflect the status of the liver, and our study has also used these liver enzymes to create novel indices to reflect the clinical status of the liver, which can be considered clinically significant and of practical value. However, toxicological studies have been conducted widely to reveal the adverse effect of rats treated with O3. For instance, rats exposed to 0.5 ppm O3 for 10 min per day can cause hepatocellular necrosis in the liver (Cretu et al. 2010), and acetaminophen can attenuate acute exposure to O3 aggravates drug-induced liver injury by delaying liver self-repair in C57BL/6 male mice (Aibo et al. 2010). The abovementioned adverse outcomes may be attributed to the oxidizing air pollutant O3-induced lipid oxidation in the body (Csallany et al. 1985) and irritate the progression of liver fibrosis.

Because of the similar pathways of outdoor and indoor air pollution entering the human body, we may consider that the association of IAP from solid fuel burning and ambient O3 exposure with hepatic fibrosis is biologically plausible. One noteworthy finding of this study was that O3 exposure and solid fuel use had a synergistic effect on intermediate-high advanced fibrosis risk. Although the evidence for the combined associations of ambient O3 and solid fuel use with advanced fibrosis risk is lacking, several epidemiological studies have shown that mixed exposure to air pollutants had adverse effects on human health. Zhang et al. (2022) found that the joint effect of multiple air pollutants (PM2.5, PM10, NO2, SO2, O3, and CO) on lipid profiles. Siddika et al. (2019) revealed a synergistic effect of prenatal exposure to PM2.5 and O3 on preterm birth (Siddika et al. 2019). Animal experiments showed that the Wistar rats who inhaled 0.8ppm ozone and PM2.5 successively exhibited inflammatory changes and pathological features in their lungs, indicating that joint exposure to different environmental pollutants may be more toxic to the lungs than exposure to a single pollutant (Wang et al. 2015). It is fact that the volume of air exchange between outdoor and indoor environments is very high in rural China (Carter et al. 2016), which increased their chance for the co-exposure of indoor and outdoor air pollution. In accordance with the abovementioned reasons, we may deduce that it is a reasonable finding that O3 exposure synergist with solid fuel use was related to increased advanced fibrosis risk.

Another notable finding was that women’s liver function was more sensitive to air pollutants, which may be due to factors such as woman’s anatomy, hormone levels, and lifestyle. It is hard to ignore the fact that 66% of the population in low- and middle-income countries are still unable to access the clean energy, and IAP remains a major contributor to indoor particulate matter exposure (Gordon et al. 2014). In fact, women tend to cook at home and are often exposed to oil fuel and the production of fuel combustion for longer time than men (Chen et al. 2020), which had been verified linked to various chronic diseases (Lin et al. 2020). A prospective cohort study of 0.5 million Chinese adults also found a consistent positive association between solid cooking fuel use and chronic digestive diseases (Wen et al. 2023). PAHs derived primarily from solid fuel use (Ambade et al. 2021c) were associated with elevated ALT levels in adolescent females (OR = 2.33, 95%CI: 1.15, 4.72) (Xu et al. 2021), suggesting that PAHs may have toxic effects on the liver. In addition, estrogen has non-reproductive effects as a regulator of the immune system, growth, nervous function, and metabolism. Especially in the liver, impaired estrogen receptor expression and function are closely associated with obesity and liver-related metabolic disorders (Ezhilarasan 2020). As mentioned above, it is reasonable to assume that women are susceptible to IAP.

Although the underlying mechanisms for the association between air pollution and hepatic fibrosis have been not entirely clarified, extensive studies were available to concern the potential mechanisms of involvement in air pollutants such as PM2.5 or O3 associated with liver dysfunction. In vitro experiments on mice have shown that exposure to PM2.5 induced a significant effect on promoting liver fibrosis. PM2.5 may induce increased nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) activity and generate excessive reactive oxygen species (ROS) by activating PINK1/parking signaling pathway to cause mitochondrial phagocytosis and ultimately lead to liver fibrosis (Qiu et al. 2019; Zheng et al. 2015). Targeted metabolomics experiments in mice have demonstrated that long-term exposure to PM2.5 aggravates liver metabolic disorders which represents a time-response relationship (Samavat et al. 2021). And PM2.5 and O3 co-exposure induced alteration of key metabolic pathways (glucose, lipids, and amino acids) and activation of oxidative damage and inflammation in the liver (Miller 2020; Samavat et al. 2021; Theis et al. 2014). These phenomena are biologically plausible, and they may be partially explained by that the cytochrome P-450 enzyme plays an important role in detoxifying the xenobiotics which may determine the production of ROS and major source from the liver (Rosen et al. 2001; Tomaru et al. 2007).

Several limitations of the present study should be noted. First, this study cannot establish causal associations between indoor or outdoor air pollution and hepatic fibrosis indices due to the nature of the cross-sectional design. However, we provide some epidemiological clues for subsequent experimental explorations. Second, FIB-4, APRI, and AST/ALT may exist in misclassification in the assessment of the degree of liver fibrosis. However, the sensitivity and specificity of FIB-4 (Kim et al. 2016), APRI (Wai et al. 2003), and AST/ALT (Hou et al. 2022) have been demonstrated to be accepted in previous studies. Third, IAP was assessed by using self-reported data on domestic fuel use in this study. However, results from a large perspective study indicate that self-reported fuel use shows a high agreement between the baseline survey and the re-survey (kappa coefficient=0.61) (Liu et al. 2019). Finally, the type of fuel used for cooking and the air quality of the environments around people may be influenced by their socio-economic status, lifestyle, and traditional customs. However, no substantial change was observed in the results of this study between unadjusted and adjusted factors.

In conclusion, high levels of O3 exposure and solid fuel use had a synergistic effect on the risk of liver dysfunction, and women are more susceptible to air pollutants. Our study may provide scientific evidence for the synergistic effects of co-exposure to indoor and outdoor air pollution on human health. It also suggests that promoting cleaner fuel use is an effective measure to maintain sustainable development of the environment and gain beneficial effect on human health.