Environmental Risk Factors for Acute Respiratory Distress Syndrome

INTRODUCTION

The acute respiratory distress syndrome (ARDS) affects an estimated 10% of patients admitted to an intensive care unit (ICU), results in a mortality of approximately 40%, and is associated with substantial morbidity.¹ Currently, the syndrome lacks pharmacologic therapies that target the underlying biology of lung injury.² Additionally, strategies aimed at preventing ARDS development and progression are lacking. A growing body of research has identified multiple environmental exposures that confer increased ARDS risk and/or ARDS mortality in those with a precipitation risk factor, such as sepsis or trauma. These environmental exposures, including ambient air pollutants, cigarette smoke, and alcohol, are potentially reversible risk factors, providing an opportunity for primary prevention in ARDS. Additionally, understanding the mechanisms by which environmental exposures influence ARDS risk and outcomes may lead to future therapeutics targeting these mechanisms of lung injury that “prime” the lung prior to the onset of ARDS. In this review, we summarize the current evidence linking environment exposures to ARDS, examine proposed biologic mechanisms, and outline future health policy and research directions aimed at reducing the burden of ARDS morbidity and mortality.

AIR POLLUTION

The adverse effects of air pollution on cardiopulmonary morbidity and mortality have been documented since the industrial revolution.^3–10 These findings ultimately formed the rationale for air quality standards in the United States and worldwide, although excess mortality continues to be demonstrated with exposure to air pollution at levels below these standards.³ Air pollution has a multitude of deleterious effects on respiratory health, including an increase in overall mortality from respiratory disease,^5,6 an increase in the incidence of severe respiratory tract infections,^4,8 and worsening of chronic pulmonary conditions such as asthma,⁹ chronic obstructive pulmonary disease,⁷ and idiopathic pulmonary fibrosis.¹⁰ Only recently, however, have epidemiologic studies examined the role of air pollution exposure on risk of ARDS despite many overlapping pathogenic mechanisms with other respiratory diseases.

Ambient air pollution is largely composed of the byproducts of the combustion of fossil fuels. Air pollutants most relevant to respiratory morbidity and mortality include carbon monoxide (CO), ozone (O₃), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and particulate matter less than 2.5 μm (PM_2.5) and 10 μm in diameter (PM₁₀).¹¹ Low-to-moderate levels of ambient air pollutants may not lead directly to acute respiratory failure, but rather “prime” the lung to confer greater susceptibility in the setting of a second insult (Fig. 1).¹² There are several mechanisms by which air pollution may increase susceptibility to ARDS. O₃ and PM_2.5 exposure results in oxidative stress in mice and subsequent induction of inflammatory cytokines and apoptosis of pulmonary macrophages.^13,14 Additionally, low-level, longer term exposure to PM in mice exhibits similar consequences, including direct oxidative injury, increased inflammatory cells in bronchoalveolar lavage (BAL) fluid, and induction of cellular apoptosis.¹⁵ Human studies corroborate findings connecting ambient pollutant exposure, oxidative stress, and heightened inflammation to pollutant exposure, specifically observing increased neutrophilia on BAL, fractional inhaled nitric oxide, serum C-reactive protein, and circulating proinflammatory cytokines in exposed healthy adults.^16–19 Several measures of activated coagulation, epithelial permeability, and endothelial injury also exhibit an association with ambient air pollution exposure in healthy adults.^16,18–20

Fig. 1.

Lung “priming” by risk factors increases susceptibility to acute lung injury following precipitating events. (Created with BioRender.com)

Recently, a growing body of evidence has established an epidemiologic association between air pollution and ARDS risk (Table 1). A prospective cohort of critically ill patients in Tennessee identified a dose-dependent relationship between exposure to ambient O₃, as determined by local Environmental Protection Agency (EPA) air quality monitors, and the development of ARDS.²¹ In this cohort, the relationship between O₃ exposure and risk of ARDS was modified by cigarette use and etiology of ARDS, with O₃ having the greatest impact on ARDS risk among patients with a history of cigarette use and trauma. A second prospective cohort of critically ill patients with trauma in the Philadelphia region found that low-to-moderate long-term exposure to O₃, in addition to NO₂, SO₂, and PM_2.5, were independently associated with increased odds of developing ARDS.²² This investigation added to the prior Tennessee-based study by enrolling patients in a region of the country with a higher density of EPA monitors leading to improved exposure ascertainment. A subsequent study of critically ill patients with sepsis, performed at the same Philadelphia center, also demonstrated an increased risk of ARDS after short-term and long-term exposure to SO₂, NO₂, and PM_2.5. This study further demonstrated an association between long-term pollutant exposure and increased plasma markers of inflammation measured early after presentation with sepsis.²³

Table 1.

Major studies evaluating the relationship between air pollution and acute respiratory distress syndrome

Study Authors and Year	Study Type	Population	Pollutants	Outcome
Ware et al,21 2016	Patient level	At risk for ARDS	O3, NO2, SO2, PM2.5, and PM10	Long-term O3 and NO2 → ↑ risk of ARDS
Rush et al,27 2017	Ecologic	ARDS	O3	O3 → ↑ mortality from ARDS
Lin et al,26 2018	Ecologic	ARDS	PM10, PM2.5, and PM1	Short-term PM10, PM2.5, and PM1 → ↑ incidence of ARDS
Reilly et al,22 2018	Patient level	Trauma	O3, NO2, SO2, CO, and PM2.5	Long-term O3, NO2, SO2, CO, and PM2.5 → ↑ risk of ARDS
Rhee et al,25 2019	Ecologic	ARDS	O3 and PM2.5	Long-term O3 and PM2.5 → ↑ incidence of ARDS
De Weerdt et al,28 2020	Patient level	Mechanically ventilated patients	NO2, PM2.5, PM10, and BC	NO2, PM2.5, PM10, and BC → ↑ duration of mechanical ventilation
Gutman et al,24 2022	Ecologic	ARDS	O3, NO2, SO2, and PM2.5	Long-term NO2, SO2, and PM2.5 → incidence of ARDS NO2 → ↑ mortality from ARDS
Reilly et al,23 2023	Patient level	Sepsis	O3, NO2, SO2, CO, PM2.5, and PM10	Long-term NO2, SO2, CO, PM10, and PM2.5 → ↑ risk of ARDS

Patient-level studies represent epidemiologic studies with individual-level data; ecologic studies represent population-level studies investigating the relationship between regional air pollution and ARDS incidence/mortality.

Abbreviations: BC, black carbon; CO, carbon monoxide; NO₂, nitrogen dioxide; O₃, ozone; PM₁, particulate matter with an aerodynamic diameter ≤1 μm; PM₁₀, particulate matter with an aerodynamic diameter ≤10 μm; PM_2.5, particulate matter with an aerodynamic diameter ≤2.5 μm; SO₂, sulfur dioxide.

Several large retrospective ecologic studies at the population level have similarly demonstrated associations between ambient pollution exposures and the incidence of ARDS. A retrospective cohort study in France found that the annual incidence of ARDS was elevated in regions with elevated levels of NO₂, PM_2.5, PM₁₀, and O₃.²⁴ Additionally, a study of over 1 million hospitalized Medicare beneficiaries found a statistically significant increase in the risk of hospitalization for ARDS among patients from regions with higher exposure to O₃ and PM_2.5.²⁵ Finally, a retrospective study in Guangzhou, China, identified an association between ambulance dispatches for ARDS and short-term exposure to high levels of ambient particulate matter between 1 and 10 μm in diameter.²⁶

In addition to conferring an elevated risk for the development of ARDS, several studies have demonstrated associations between exposure to air pollutants and worse outcomes. Rush and colleagues²⁷ analyzed 93,950 hospital admissions where patients underwent mechanical ventilation for ARDS from the 2011 Nationwide Inpatient Sample. Patients with ARDS from the highest O₃ pollution cities had a statistically higher mortality relative to the rest of the country after adjustment for potential confounders. Similarly, a study conducted in Belgium found an association between short-term air pollutant exposure and duration of mechanical ventilation among critically ill patients.²⁸ While not all of these patients met criteria for ARDS, the findings do suggest a link between air pollutant exposure and outcomes in acute respiratory failure.

Unfortunately, socioeconomic, racial, and ethnic disparities in respiratory health are widened by differential exposure to ambient air pollution. In the United States, average concentrations of air pollutants have improved substantially over the last several decades; however, geographic disparities among communities have not been reduced.²⁹ Air pollution concentrations remain higher in black and Hispanic communities and communities with lower socioeconomic status.^30,31 The aforementioned Philadelphia-based sepsis cohort study demonstrated a substantially greater burden of pollutant exposure among Black subjects and subjects of lower socioeconomic status.²³ Targeted public health interventions to reduce air pollution in these communities may have the greatest potential to reduce ARDS risk.

Epidemiologic studies of air pollutant exposure and ARDS have limitations. Most importantly, given the somewhat stochastic nature of precipitation factors for ARDS, exposure estimates rely on EPA monitors and geocoded home addresses rather than individual air pollutant monitors. This approach fails to account for indoor air pollutants, occupational exposures, and exposure during time away from the home. Additionally, several of the individual air pollutants are collinear, complicating identification of the causal pollutant(s). Future research may improve exposure estimates using mobile phone location data and lower cost sensors.³² Improvements in individual exposure estimates may then allow for multipollutant models to assess the most relevant pollutants impacting ARDS risk.

Air Pollution and Coronavirus Disease 2019

The coronavirus disease 2019 (COVID-19) pandemic led in a large increase in the global incidence of ARDS, resulting in millions of deaths and significant excess morbidity.³³ The most common manifestation of severe COVID-19 in the ICU is ARDS, and unsurprisingly, several risk factors for traditional ARDS are also associated with ARDS in COVID-19.^34–36 Priming by pollutants for lung injury related to COVID-19 likely occur via the aforementioned mechanisms of oxidative stress, lung epithelial injury, inflammation, and endothelial dysfunction. Additionally, air pollution exposure may lead to overexpression of the angiotensin converting enzyme 2, a key receptor for viral entry present on the surfaces of the respiratory tract, and subseqeunty augmented COVID-19 infection.³⁷

Multiple large ecologic studies have identified associations among chronic air pollutant exposure and COVID-19 infection, severity, and mortality.³⁸ Ecologic studies, however, are limited as they are unable to account for individual-level differences and important confounders such as socioeconomic status. More recently, case–control and cohort studies utilizing individual-level data have been reported^39–43 with varying quality and confounder adjustment. A recent meta-analysis of individual-level studies identified associations between air pollutant exposures and COVID-19 incidence and severity, as well as a nonsignificant trend toward increased mortality. Unfortunately, many of the included studies had methodological issues including failure to adjust for socioeconomic status. Despite these limitations, growing evidence exists linking air pollutant exposures to COVID-19 disease, further supporting a complex interplay among air pollution, pathogen infection, and acute lung injury.

Wildfire Pollution

Exposure to wildfire smoke represents a unique subset of air pollution-associated respiratory hazards as the exposure concentration and chemical makeup of wildfire smoke are distinct from air pollution generated by the combustion of fossil fuels.⁴⁴ As wildfire events become more severe, prolonged, and ubiquitous as the result of climate change, there is a growing urgency to understand the effect on respiratory health. Specific epidemiologic studies examining the association between wildfire smoke exposure and ARDS incidence are currently lacking. However, several experimental and observational studies support mechanistic plausibility. In vitro studies of alveolar epithelial cells exposed to soluble wood smoke demonstrate impairment in barrier function and increased oxidative stress-induced apoptosis.^45,46 Additionally, wildfire exposure events acutely impact lung function, leading to decreases in forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), and FEV1/FVC ratio, as well as sustained impacts on long-term lung function.⁴⁷ Wildfire exposure has been linked epidemiologically to an increased risk of hospitalization in patients with obstructive lung disease,⁴⁸ risk of influenza,⁴⁹ and COVID-19 hospitalizations and mortality.⁵⁰ Future research focused on the acute and chronic impact of wildfire exposure on ARDS incidence and severity is needed.

CIGARETTE SMOKE

Worldwide, 8.7 million deaths annually are attributable to tobacco smoke, 1.3 million of which result from environmental tobacco exposure among people who do not use tobacco.⁵¹ Despite successful policy interventions to decrease morbidity and mortality related to cigarette smoking,⁵¹ tobacco use remains the leading cause of preventable death worldwide, and accounts for approximately 1 in every 5 deaths in the United States each year.^51,52 Although rates of tobacco use are decreasing over time, an estimated 11.5% of adults in the United States continue to smoke cigarettes.⁵²

Observational studies investigating the impact of tobacco on ARDS risk have identified associations between exposure to cigarette smoke and the development of ARDS following an array of predisposing insults. Christenson and colleagues⁵³ first identified active smoking as an independent risk factor for ARDS in a retrospective cohort of over 3800 patients undergoing cardiopulmonary bypass. Iribarren and colleagues⁵⁴ similarly identified an association between cigarette use and ARDS in a retrospective analysis of an administrative database of over 121,000 patients. A prospective case-controlled study found that active smoking conferred an increased risk for the development of transfusion-associated lung injury.⁵⁵ Additionally, a multicenter prospective cohort study identified an association between donor smoking history and the development of primary graft dysfunction, a form of acute lung injury, after lung transplantation.⁵⁶

Notably, one large prospective multicenter cohort study of over 5000 hospitalized patients at risk for ARDS failed to identify tobacco use, as reported by patients or their surrogates, as a risk factor for ARDS.⁵⁷ The conflicting results of these observational studies may, in part, be attributable to misclassification of cigarette smoke exposure,⁵⁸ which can be particularly challenging in the setting of critical illness.⁵⁹ In order to address this issue, Calfee and colleagues measured plasma cotinine, a nicotine metabolite that has been validated as a stable biomarker of exposure to cigarette smoke,⁶⁰ and determined its association with ARDS among patients with severe blunt trauma.⁶¹ This study identified an association between plasma cotinine and development of ARDS, which was notably independent of whether cigarette smoke exposure was active or passive. This has since been validated in a larger, prospective cohort.⁶² Studies have also leveraged urinary biomarkers, which are reflective of a longer duration of cigarette exposure and thus reduce exposure misclassification, to further understand the impact of smoking on ARDS.^63–65 Finally, a recent meta-analysis of over 4000 patients corroborated cigarette exposure as a risk factor, with an odds ratio of 1.80 (95% confidence interval 1.20–2.43).⁶⁶ Paradoxically, there is some evidence to suggest that exposure to cigarette smoke is associated with a lower mortality in ARDS when compared to nonsmokers^64,65; however, large biomarker-based studies suggest cigarette smokers are younger, healthy, and have a lower severity of illness, possibly explaining these earlier findings.⁶⁷

There are several proposed mechanisms by which chronic exposure to cigarette smoke confers an increased risk of lung injury, including impaired mucociliary clearance, decreased immunoglobulin A-mediated mucosal immunity, decreased serum immunoglobulin levels, diminished alveolar macrophage function, blunted T-cell signaling, and diminished cytokine production.^68,69 Healthy volunteers with a history of active cigarette smoking have higher serum and BAL marker levels of epithelial permeability, endothelial dysfunction, and inflammation compared to nonsmokers.⁷⁰ Additionally, cigarette smoke has particularly deleterious effects on immunity against specific bacterial and viral infections of the lung, including Pseudomonas aeruginosa, Haemophilus influenzae, influenza, and COVID-19.^71–77 While the data behind an association between cigarette smoke exposure and COVID-19 risk and outcomes remain unclear, a causal role for cigarette smoking in COVID-19 was suggested by a Mendelian randomization study using the UK Biobank.⁷⁸

Electronic Cigarettes

“Vaping” refers to the inhalation of aerosolized products following the heating of a liquid via portable electronic delivery systems. Products commonly contain nicotine, cannabinoids, humectants (aerosolizing agents), synthetic flavors, and other additives.⁷⁹ Electronic cigarettes (“e-cigarettes”) were introduced to the market in the United States in 2007, and vaping delivery devices gained popularity in the ensuing years.^80–82 E-cigarette or vaping use-associated lung injury (EVALI) was first recognized in 2019,⁷⁹ although the occurrence likely preceded this report. Awareness of EVALI quickly followed, with over 2000 hospitalizations and 68 deaths reported to the Center for Disease Control between August 2019 and January 2020.⁷⁹

Clinically, ARDS was present in 63% of the fatal EVALI cases reported.⁸³ A systematic review of the imaging characteristics of 184 patients with EVALI identified bilateral infiltrates as the most common radiographic finding, present in 41% of cases.⁸⁴ Additionally, the pathologic correlate to ARDS, diffuse alveolar damage, in seen on lung biopsy specimen from patients diagnosed with EVALI.⁸⁵ The pathophysiology of ARDS in the setting of EVALI remains poorly understood, in large part due to the diversity of causative agents and heterogeneity of the resulting lung injury. Vitamin E acetate (VEA), a common additive in cannabinoid vaping products, has the highest degree of evidence of harm. VEA was recovered in 94% to 100% of BAL samples from patients with EVALI.⁸⁶ Evidence of pulmonary toxicity was furthered by animal studies, in which mice exposed to aerosolized VEA demonstrated increased leukocyte recruitment, BAL protein, proinflammatory cytokines, and lipid-laden macrophages than controls.^87,88 The mechanism of injury is not fully understood, but may be related to the production of toxic ketene and benzene gas following thermal decomposition of VEA at high temperatures.⁸⁹ In vitro studies of several additional common components of vaping products suggest pulmonary toxicity, including humectants and flavoring chemicals.^88,90,91 Enhanced investigation of the components of vaping products and their effects on short-term and long-term pulmonary health is essential to inform a public health response.

EVALI is a diagnosis of exclusion made in the absence of additional insults that account for ARDS. However, the potential for vaping to induce subacute to chronic changes that “prime” the lung for the development of ARDS following a second insult remains underexplored. Mice exposed to electronic cigarette vapor for 4 months did not develop acute lung injury but did exhibit alterations in phospholipid metabolism crucial to the integrity of alveolar surfactant.⁹² In addition to metabolic effects, vaping products may also increase susceptibility to infection. In vivo, e-cigarette vapor has been shown to inhibit epithelial barrier cell function and increase susceptibility to respiratory syncytial virus.^93,94 Future research should investigate the potential for an increased ARDS risk among patients who regularly use e-cigarettes and vaping products.

ALCOHOL

Alcohol use disorder affects 11.2% of adults in the United States and is responsible for an estimated 140,000 deaths annually.⁹⁵ Several studies have identified chronic alcohol use as a risk factor for ARDS in various populations^55,57,96,97 (Table 2). Similarly, a meta-analysis found that alcohol increased the odds of ARDS among those at risk, with an odds ratio of 1.89 (95% CI 1.45–2.48).⁹⁸ In contrast, a retrospective study of over 120,000 patients failed to identify an association between self-reported alcohol use and ARDS.⁵⁴ Methodologic differences in exposure definition may account for these conflicting findings. Prospectively evaluating alcohol use with a validated instrument administered by study investigators may improve exposure classification.⁹⁹ The definition of alcohol exposure may be further refined by blood markers such as phosphatidylethanol (PEth). PEth exhibits promising test characteristics for identifying chronic excessive alcohol use¹⁰⁰; however, it is limited by differential performance by race, body mass index, hemoglobin concentration, and liver function.¹⁰¹

Table 2.

Epidemiologic studies evaluating the relationship between alcohol use and acute respiratory distress syndrome

Study Authors and Year	Population	Exposure	Alcohol as a Risk Factor for ARDS?	Alcohol as a Risk Factor for Mortality in ARDS?
Moss et al,96 1996	At risk for ARDS	Medical record review	Yes (RR 1.98)	Yes (OR 6.26)
Iribarren et al,54 2000	Kaiser Permanente Health Plan subscribers	Self-reported alcohol use upon health plan enrollment	No
Licker et al,97 2003	Patients undergoing pneumonectomy	Medical record review	Yes (OR 1.9)	—
Moss et al,99 2003	Septic shock	Study personnel administration of the Short Michigan Alcohol Screening Test	Yes (OR 3.68)	No
Gajic et al,57 2011	At risk for ARDS	Medical record review	Yes	—
Clark et al,102 2013	Patients enrolled in ARDS-Network trials	Study personnel administration of the AUDIT		Yes (OR 1.80)

Abbreviations: OR, odds ratio; RR, relative risk.

Initial prospective cohort studies examining the impact of chronic alcohol use on ARDS mortality yielded conflicting results^96,99 (see Table 2). To further investigate the impact of chronic alcohol use on outcomes of ARDS, Clark and colleagues¹⁰² performed a secondary analysis of over 1000 patients enrolled in 3 randomized controlled trials of ARDS, all of which prospectively collected information on alcohol use via the Alcohol Use Disorders Identification Test (AUDIT). They identified a U-shaped relationship between strata of alcohol use and mortality from ARDS, and they identified a greater risk of death from ARDS among those with severe alcohol misuse when compared to those with low-risk alcohol consumption. Notably, participants who did not use alcohol were found to have a higher burden of baseline comorbidities. The investigators posit that this may account for the appearance of a protective effect of mild alcohol use from death among patients with ARDS, as well as the disparate results of earlier studies.

As is the case with exposure to cigarette smoke, chronic alcohol is hypothesized to “prime” the lung to increase susceptibility to ARDS following a precipitating insult via immunomodulatory mechanisms^103,104 and aberrations in alveolar epithelial permeability.¹⁰⁵ Specifically, the depletion of pulmonary glutathione, an antioxidant present in the alveolar epithelium, is a plausible mechanism by which chronic alcohol use predisposes the lung to injury.¹⁰⁶ It may also be the case that the impact of chronic alcohol use on delirium and critical illness as a whole portends a worse prognosis from ARDS.¹⁰²

SUMMARY

The imperative to reduce exposure to air pollution, cigarette smoke, and alcohol is urgent and incontrovertible. In light of the high mortality associated with ARDS and the absence of effective pharmacotherapeutics, upstream primary prevention represents a critically important strategy to reduce deaths. Continued public health efforts to raise air quality standards, reduce primary and secondary exposure to cigarette smoke, and address chronic alcohol use should be prioritized in order to reduce morbidity and mortality both among those at risk for critical illness and the public at large.

Future research may also enhance our understanding of the mechanistic relationship between chronic exposure to environmental hazards and vulnerability to acute insults. The precise cellular and immunomodulatory mechanisms by which these exposures prime the lung to acute injury remain unclear, but may be reversible. Elucidating these pathways may reveal opportunities for targeted treatment in those at risk for or who have developed ARDS. Additionally, further refinement of each exposure definition, ranging from biomarkers such as cotinine and PEth to precise individual-level data on air pollution exposure, will continue to enhance our epidemiologic understanding of these environmental risk factors.

Finally, environmental hazards are dynamic, and further research will be necessary to characterize and address new and emerging threats. Climate change will inevitably alter the environmental milieu, and the recent global increase in wildfire smoke exposure has underscored the need for additional investigation. In conclusion, there has been significant progress over the last several decades in understanding and mitigating environmental risk factors for ARDS. Future research, as well as strong public health policy interventions, have the potential to build upon this progress in preventing morbidity and mortality from ARDS.

KEY POINTS.

• Environmental exposures, including air pollution and cigarette smoke, increase susceptibility to acute respiratory distress syndrome (ARDS).

• Research elucidating pathways by which environmental exposures prime the lung for injury may reveal therapeutic targets.

• In the absence of existing therapeutics for ARDS, upstream prevention by public policy interventions is a promising strategy to reduce ARDS-related morbidity and mortality.

CLINICS CARE POINTS.

• Environmental risk factors for ARDS, including air pollution, cigarette smoke, and alcohol, are thought to “prime” the lung and increase susceptibility to acute lung injury following precipitating events, such as sepsis and trauma.

• Exposure to ambient air pollution, largely composed of byproducts from the combustion of fossil fuels, is associated with an elevated risk of developing ARDS, as well as worse outcomes from ARDS. Socioeconomic, racial, and ethnic disparities in respiratory health are widened by differential exposure to air pollution.

• Exposure to cigarette smoke is associated with an increased risk of ARDS. E-cigarette or vaping use-associated lung injury (EVALI) commonly presents as ARDS, although the pathophysiology is incompletely understood, in part due to the diversity of causative agents and heterogeneity of the resulting lung injury.

• Alcohol use has been identified as a risk factor for the development of ARDS. Studies investigating the effect of alcohol use on outcomes among patients with ARDS have yielded conflicting results, perhaps due to the high burden of comorbidities among patients that did not use alcohol in these studies.

• Environmental risk factors represent a promising target to address the high morbidity and mortality associated with ARDS. Further research investigating the mechanism by which environmental hazards predispose to ARDS may reveal therapeutic targets. Public policy that aims to mitigate exposure to air pollution, cigarette smoke, and alcohol represent a critically important strategy for upstream primary prevention of ARDS.