Important Indoor Air Pollutants Research Articles

Study on revision of standard limits for nitrogen dioxide in "Standards for indoor air quality（GB/T 18883-2022）" in China

Nitrogen dioxide (NO2) is an important indoor air pollutant, with both outdoor and indoor sources contributing to indoor NO2 exposure levels. Considering the association of high NO2 exposure with adverse health effects, the Standards for indoor air quality (GB/T 18883-2022) have been revised to further restrict indoor NO2 limit values. The 1-h average concentration limit value for NO2 has been reduced from 0.24 mg/m3 to 200 μg/m3.This study analyzed the technical contents related to the determination of the limits of indoor NO2 in Standards for Indoor Air Quality (GB/T 18883-2022), including source, exposure level, health effects, and the process and evidence basis for determining the limit value. It also proposed prospects for the direction for the implementation of the indoor NO2 standard.

Laboratory Performance Evaluation of a Low-Cost Electrochemical Formaldehyde Sensor.

Formaldehyde is a known human carcinogen and an important indoor and outdoor air pollutant. However, current strategies for formaldehyde measurement, such as chromatographic and optical techniques, are expensive and labor intensive. Low-cost gas sensors have been emerging to provide effective measurement of air pollutants. In this study, we evaluated eight low-cost electrochemical formaldehyde sensors (SFA30, Sensirion®, Staefa, Switzerland) in the laboratory with a broadband cavity-enhanced absorption spectroscopy as the reference instrument. As a group, the sensors exhibited good linearity of response (R2 > 0.95), low limit of detection (11.3 ± 2.07 ppb), good accuracy (3.96 ± 0.33 ppb and 6.2 ± 0.3% N), acceptable repeatability (3.46% averaged coefficient of variation), reasonably fast response (131-439 s) and moderate inter-sensor variability (0.551 intraclass correlation coefficient) over the formaldehyde concentration range of 0-76 ppb. We also systematically investigated the effects of temperature and relative humidity on sensor response, and the results showed that formaldehyde concentration was the most important contributor to sensor response, followed by temperature, and relative humidity. The results suggest the feasibility of using this low-cost electrochemical sensor to measure formaldehyde concentrations at relevant concentration ranges in indoor and outdoor environments.

Enhanced Adsorption of Aromatic Volatile Organic Compounds on a Perchloro Covalent Triazine Framework through Multiple Intermolecular Interactions.

Volatile organic compounds (VOCs) may have short- and long-term adverse health effects. Especially, aromatic VOCs including benzene, toluene, ethylbenzene, and xylene (BTEX) are important indoor air pollutants. Developing highly efficient porous adsorbents with broad applicability remains a major challenge. In this study, a perchlorinated covalent-triazine framework (ClCTF-1-400) is prepared for adsorbing BTEX. ClCTF-1-400 is confirmed as a partially oxidized/chlorinated microporous covalent triazine framework through a variety of characterization. It is found that ClCTF-1-400 is reversible VOCs absorbent with very high absorption capacities, which can adsorb benzene (693mg g-1 ), toluene (621mg g-1 ), ethylbenzene (603mg g-1 ), o-xylene (500mg g-1 ), m-xylene (538mg g-1 ), and p-xylene (592mg g-1 ) at 25 °C and their saturated vapor pressure (≈ 1kPa). ClCTF-1-400 is of higher adsorption capacities for all selected VOCs than activated carbon and other reported adsorbents. The adsorption mechanism is also inferred through theoretical calculation and in-site Fourier Transform Infrared (FTIR) spectroscopy. The observed excellent BTEX adsorption performance is attributed to the multiple weak interactions between the ClCTF-1-400 frameworks and aromatic molecules through multiple weak interactions (CH… π and CCl… π). The breakthrough experiment demonstrates ClCTF-1-400 has the potential for real VOCs pollutant removal in air.

Perovskite Oxides in Catalytic Combustion of Volatile Organic Compounds: Recent Advances and Future Prospects

Volatile organic compounds are a kind of important indoor and outdoor air pollutants. In recent years, more and more attention has been paid to the ways of volatile organic compound elimination because of its potential long‐term effects on human health. Among the various available methods for volatile organic compound elimination, the catalytic combustion is the most attractive method due to its high efficiency, low cost, simple operation, and easy scale‐up. Perovskite oxides, as a large family of metal oxides with their A‐site mainly of lanthanide element and/or alkaline earth metal element and B‐site of transition metal element, have been extensively investigated as active and stable catalysts for volatile organic compound removal reactions due to their abundant compositional elements, high thermal/chemical stability, and compositional/structural flexibility. The catalytic performance of perovskite oxides is strongly depended on its material composition, morphology, and surface/bulk properties, while the doping, tailored synthesis route, and composite construction may have a significant effect on the bulk (oxygen vacancy concentration, lattice structure), surface (oxygen species, defect) properties, and particulate morphology, consequently the catalytic activity and stability for volatile organic compound removal. Herein, a comprehensive review about the recent advances in perovskite oxides for volatile organic compound elimination reactions based on catalytic combustion is presented from different aspects with a special emphasis on the material design strategies, such as compositional tuning, morphology control, nanostructure building, hybrid construction, and surface modification. At last, some perspectives are presented on the development and design of perovskite oxide‐based catalysts for volatile organic compound removal applications by highlighgting the critical issues and challenges.

Climate and occupancy based investigation of air pollutants in office spaces

Indoor air quality (IAQ) has long been a matter of public concern as many people are spending a large portion of their time indoors. The concentration of some pollutant in offices and other work places can often be 2 to 5 times higher than typical outdoor concentrations. This study investigates the relationship between office space available per employee and outside weather condition with indoor pollutants level at 25 office locations of a large international architecture and design firm. Air pollutants measured in this study included carbon dioxide (CO2), particulate matter (PM2.5), and volatile organic compounds (VOCs). They have been monitored for a three-month period from December 1st, 2017 to February 28th, 2018 in 23 offices located in the U.S., one office in China, and one office in Dubai. The first phase of this study analyzed the hourly average levels of mentioned pollutants at these offices during working and non-working hours. Then those levels were correlated to the number of employees at all of the locations. The second phase observed the difference in CO2 level, as the most important indoor air pollutant, related to outside weather condition (temperature and humidity) at three locations in the U.S. The initial result of phase one proved the linear relationship between the number of employees and CO2 levels in office buildings. It concluded, that the low area per employee rate will cause the high CO2 concentrations. Moreover, a comparison between indoor CO2 level and outside weather conditions (temperature and humidity) in the second phase at three office shows that CO2 level are affected directly by outdoor conditions during working hours. Based on the building climate zone, when temperature and humidity are far from the comfort zone and mechanical ventilation system are operated, CO2 concentration could variously change during working hours.

Enhanced Formaldehyde Removal from Air Using Fully Biodegradable Chitosan Grafted β-Cyclodextrin Adsorbent with Weak Chemical Interaction.

Formaldehyde (HCHO) is an important indoor air pollutant. Herein, a fully biodegradable adsorbent was synthesized by the crosslinking reaction of β-cyclodextrin (β-CD) and chitosan via glutaraldehyde (CGC). The as-prepared CGC showed large adsorption capacities for gaseous formaldehyde. To clarify the adsorption performance of the as-synthesized HCHO adsorbents, changing the adsorption parameters performed various continuous flow adsorption tests. It was found that the adsorption data agreed best with the Freundlich isotherm, and the HCHO adsorption kinetic data fitted well with the pseudo second order model. The breakthrough curves indicated that the HCHO adsorbing capacity of CGC was up to 15.5 mg/g, with the inlet HCHO concentration of 46.1 mg/m3, GHSV of 28 mL/min, and temperature of 20 °C. The regeneration and reusability of the adsorbent were evaluated and CGC was found to retain its adsorptive capacity after four cycles. The introduction of β-CD was a key factor for the satisfied HCHO adsorption performance of CGC. A plausible HCHO adsorption mechanism by CGC with the consideration of the synergistic effects of Schiff base reaction and the hydrogen bonding interaction was proposed based on in situ DRIFTS studies. The present study suggests that CGC is a promising adsorbent for the indoor formaldehyde treatment.

Cigarette smoke induces ROS mediated autophagy impairment in human corneal epithelial cells

Cigarette smoke is an important indoor air pollutant which has deleterious effects on human health. Continued daily exposure to cigarette smoke has been attributed to the risk factor of ocular surface diseases. However, the mechanisms underlying the ocular surface damage are not fully elucidated. In this study, exposure to cigarette smoke extract (CSE) induced a dose- and time-dependent cytotoxicity in human corneal epithelial (HCE) cells, supported by the observation of reduced cell viability, increased apoptotic cells, elevated intracellular oxidative stress and loss of mitochondrial transmembrane potential. In addition, CSE exposure led to the impairment of proteostasis and autophagy, which resulted in the accumulation of ubiquitinated proteins as aggregates in peri-nuclear spaces. Furthermore, the autophagy inducer, cysteamine was shown to attenuate the CSE induced cell damage, oxidative stress and mitochondrial dysfunction in HCE cells. Moreover, cysteamine inhibited the formation of ubiquitin-positive aggregates around the peri-nuclear region, through regulating the autophagic activity of HCE cells. Similar to in vitro experiments, cigarette smoke induced proteostasis and autophagy impairment in corneal epithelial cells could be rescued by cysteamine in a cigarette smoke-exposed murine model. Therefore, this study may provide first evidence that dysfunction of autophagy contributes to the pathogenesis of ocular surface diseases associated with cigarette smoke exposure. Besides, it also suggests the potential therapeutic value of cysteamine in the prevention and treatment of cigarette smoke induced ocular surface injury.CSE induces cytotoxicity and accumulation of ubiquitinated proteins in HCE cells due to impairment of proteostasis and autophagy, which can be rescued by cysteamine.

Formaldehyde removal by the internal wall paint – preliminary study

Formaldehyde is an important indoor air pollutant. It is released mainly from building materials and consumer products. This contaminant has been recognized as having negative effect on humans. Recently, there are developed methods of reducing its emission as well as removal from indoor air. An example of the last approach is the functionalisation of interior wall paints. The influence of such paint on indoor air was preliminarily examined in this work. The obtained results show that, during intermittent exposures, responses of formaldehyde measurement device were lower in the room painted with functionalised paint compared with the room painted using traditional formula paint. This outcome was statistically significant in 67% of the examined cases at the significance level α=0.01. It was found that the serious problem associated with determining the influence of functionalised paint on indoor air, in real conditions, is the low level of formaldehyde concentrations, compared with the detection limit and accuracy of the available measurement devices.

A Colorimetric Chemical Sensing Platform for Real-Time Monitoring of Indoor Formaldehyde

Formaldehyde is one of the most important indoor air pollutants affecting human immune system, and causing various respiratory diseases and even certain cancers. Traditional formaldehyde detection technologies are bulky, expensive and difficult to maintain. Here, we report a colorimetric chemical sensing platform for fast (2 min) and sensitive detection (30 ppbv) of formaldehyde over a wide dynamic range (0-750 ppbv). The sensor can tolerate humidity variation from 5% to 90% and is immune of common interferents, such as CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (4%), SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (10 ppm), NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (125 ppbv), and O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (300 ppbv) in air. We also demonstrated continuous detection of formaldehyde on this sensing platform using disposable sensor chip.

Smoke exposure as a risk factor for asthma in childhood: a review of current evidence.

Asthma is a common chronic multifactorial disease that affects >300 million people worldwide. Outdoor and indoor pollution exposure has been associated with respiratory health effects in adults and children. Smoking still represents a huge public health problem and millions of children suffer the detrimental effects of passive smoke exposure. This study was designed to review the current evidences on exposure to passive smoke as a risk factor for asthma onset in childhood. A review of the most recent studies on this topic was undertaken to provide evidence about the magnitude of the effect of passive smoking on the risk of incidence of asthma in children. The effects of passive smoking are different depending on individual and environmental factors. Environmental tobacco smoke (ETS) is one of the most important indoor air pollutants and can interact with other air pollutants in eliciting respiratory outcomes during childhood. The increased risk of respiratory outcomes in children exposed to prenatal and early postnatal passive smoke might be caused by an adverse effect on both the immune system and the structural and functional development of the lung; this may explain the subsequent increased risk of incident asthma. The magnitude of the exposure is quite difficult to precisely quantify because it is significantly influenced by the child's daily activities. Because exposure to ETS is a likely cause for asthma onset in childhood, there is a strong need to prevent infants and children from breathing air contaminated with tobacco smoke.

Application of MCN-1 to the Adsorptive Removal of Indoor Formaldehyde

In this study, mesoporous carbon nitride (MCN-1) was synthesized by using mesoporous silica SBA-15 as a template and applied, for the first time, to the adsorption of 1-ppm formaldehyde, an important indoor air pollutant. Ammonia treatment was conducted at 700 oC for additional change in the surface characteristics of MCN-1 (i.e., increase in the quantity of nitrogen functional groups). The formaldehyde adsorption efficiencies of MCN-1 and ammonia treated MCN-1 (MCN-1-NH3) were compared with that of a nitrogen-free mesoporous carbon material (CMK-3). Adsorption experiments were conducted using a vaccumed 10-L aluminum bag. The adsorbents were characterized by X-ray diffraction, N2 adsorption-desorption, and X-ray photoelectron spectroscopy. The order of the formaldehyde adsorption efficiency of the adsorbents was MCN-1-NH3 > MCN-1 > CMK-3. The higher performance of MCN-1 than that of CMK-3 was attributed to the nitrogen containing functional groups. MCN-1-NH3 showed the highest adsorption efficiency due to the sufficient amount of nitrogen containing functional groups and large specific surface area resulting from the micropores formed during the ammonia treatment procedure.

Particulate matter investigation in indoor environment

Particulate matter is one of the most important indoor air pollutants involved in a number of&#x0D; adverse health effects, such as premature deaths and increased mortality of infants and other&#x0D; parts of sensitive population.&#x0D; This paper focuses on investigation of metal substances of suspended as well as settled&#x0D; particulate matter in indoor environment. The monitoring of particulate matter concentration&#x0D; was carried out in three rooms of a selected flat building in the city of Košice, Slovakia. The&#x0D; sampling of settled particulate matter was carried out by passive methods during the period of&#x0D; 28 days. The investigation of suspended particulate matter investigation focused on total&#x0D; suspended particles (TSP) and thoracic fraction called PM10. The presence of selected metals&#x0D; in the samples was detected by atomic absorption spectrometry.&#x0D; The surface concentrations of settled particulate matter were detected in the range from&#x0D; 276.43 μg cm-2 to 570.70 μg cm-2, mass concentrations of total suspended particulate matter&#x0D; from 59.028 to 114.583 μg m-3. PM10 concentration values reached about half of the TSP&#x0D; concentration values (PM10/TSP ratio was from 0.48 to 0.6). Higher percentage of metals&#x0D; was detected in suspended particulate matter in comparison to the settled particles. Higher&#x0D; values were detected for all metals (except iron and zinc) in each measured room.&#x0D;

The burden of disease related to indoor air in the Netherlands: do different methods lead to different results?

BackgroundDutch policy makers needed a knowledge base for prioritising control of different indoor air pollutants. Several burden of disease (BoD) estimates were available, but it was not known if they...

Indoor Particulate Matter in Different Residential Areas of Alexandria City, Egypt

Particulate matter is one of the most important indoor air pollutants that could cause a number of adverse health effects. Quantification of total suspended particulate matter (TSP) in homes in the city of Alexandria was carried out. In the present study, 15 of the 21 homes showed a higher average indoor TSP concentration than the outdoor sample, and the median indoor/outdoor ratio was 1.08. Moreover, the outdoor and the mean indoor TSP concentrations were found to correlate significantly. Concentrations of indoor particulate matter were found to be influenced by indoor sources and human activities as well as outdoor particles.

Gaseous Nitrogen Oxide Promotes Human Lung Cancer Cell Line A549 Migration, Invasion, and Metastasis via iNOS-Mediated MMP-2 Production

Gaseous nitrogen oxide (gNO) is an important indoor and outdoor air pollutant. Many studies have indicated gNO causes lung tissue damage by its oxidation properties and free radicals. However, there are considerably few data on the association between lung cancer and gNO exposure. The purpose of this study was to examine whether gNO could contribute to the process of malignant progression of lung cancer. The results of wound-healing assay and in vitro transwell assay revealed that gNO-induced dose and time dependently the migration and invasion of A549 cells, a human lung cancer cell line, under noncytotoxic concentrations. gNO was able to induce release of NO from A549 cells, an effect that was mediated via the activation of inducible nitric oxide synthases (iNOS), but not constitutive isoforms, during the same treatment period. An increased expression of matrix metalloproteinase (MMP) and a coincided reduction in repress tissue inhibitors of metalloprotease-2 were observed upon the treatment of gNO. The gNO-mediated MMP-2 induction appeared to be a consequence of nuclear factor kappa B and activation protein-1 activation, because that their DNA binding activity was enhanced by gNO. All these influences of gNO were efficiently repressed by the pretreatment of a NOS inhibitor (N(G)-nitro-L-arginine methyl ester). Using a mouse model, we showed that gNO promoted A549 metastasis to the lung through a mechanism involving the iNOS-dependent MMP-2 activity. Our data imply that gNO exposure, which in turn led to iNOS activation and the enhancement of MMP-mediated cellular events, was related to lung cancer development.

Endotoxins in tobacco smoke: Shifting tobacco industry positions

In the 1980s, the tobacco industry started a campaign to divert attention away from secondhand tobacco smoke (SHS) as a major source of indoor air pollution in workplaces by highlighting the roles of other indoor air pollutants. The industry, working through "third parties," highlighted endotoxins, naturally occurring substances that cause numerous inflammatory reactions in humans, as an alternative explanation to SHS as causing indoor air problems. In 1995, Hasday and colleagues were the first to present findings that cigarette smoke contains significant quantities of endotoxins. This discovery surprised tobacco industry scientists. The 1999 publication of the full Hasday et al. findings received only limited media attention but got the full attention of Philip Morris scientists concerned about a new public health issue and a new basis for regulation of workplace smoking by the U.S. Occupational Safety and Health Administration, which already regulated workplace endotoxin exposures from other sources. Philip Morris undertook an internal endotoxin research project to test the Hasday et al. findings and to determine if endotoxin-free cigarettes were possible. Although experiments were conducted to remove endotoxin from the tobacco, there is no evidence that they were successful. Following confirmation of SHS as an important source of endotoxins, the scientist promoting endotoxins as an important indoor air pollutant for the tobacco industry softened his position on the role of endotoxins as indoor pollutants. The presence of endotoxins in SHS provides an additional mechanism for the adverse effects of SHS that should be researched further, and the risk of exposure should be assessed.

Which ornamental plant species effectively remove benzene from indoor air?

Phytoremediation—using plants to remove toxins—is an attractive and cost effective way to improve indoor air quality. This study screened ornamental plants for their ability to remove volatile organic compounds from air by fumigating 73 plant species with 150 ppb benzene, an important indoor air pollutant that poses a risk to human health. The 10 species found to be most effective at removing benzene from air were fumigated for two more days (8 h per day) to quantify their benzene removal capacity. Crassula portulacea, Hydrangea macrophylla, Cymbidium Golden Elf., Ficus microcarpa var. fuyuensis, Dendranthema morifolium, Citrus medica var. sarcodactylis, Dieffenbachia amoena cv. Tropic Snow; Spathiphyllum Supreme; Nephrolepis exaltata cv. Bostoniensis; Dracaena deremensis cv. Variegata emerged as the species with the greatest capacity to remove benzene from indoor air.

Gaseous nitrogen oxide repressed benzo[a]pyrene-induced human lung fibroblast cell apoptosis via inhibiting JNK1 signals

Benzo[a]pyrene (B[a]P) is present in environmental pollution and cigarette smoke. B[a]P has been shown to induce apoptosis in hepatoma cells, human B cells, human ectocervical cells, macrophages, and rat lungs. Nitrogen oxides (NOx) are the other important indoor and outdoor air pollutants. Many studies have indicated that NO gas causes lung tissue damage both by its oxidative properties and free radicals. In our previous study we demonstrated that NO gas induced proliferation of human lung fibroblast MRC-5 cells. In this study we showed that NO gas inhibits B[a]P-induced MRC-5 cells apoptosis by cell cycle analysis. Western blot data revealed that NO gas increased the expressions of anti-apoptosis proteins (Bcl-2 and Mcl-1) and decreased the expression of apoptosis proteins (Bax, t-Bid, cytochrome c, FasL, and caspases) after B[a]P treatment. We further clarified that B[a]P-induced MRC-5 cell apoptosis via JNK1/FasL and JNK1/p53 signals. In conclusion, NO gas inhibited B[a]P-induced MRC-5 cells apoptosis via inhibition of JNK1 apoptosis pathway and induction of Bcl-2 and Mcl-1 anti-apoptosis pathway.

Child Health and the Environment

In developed countries, people spend around 90% of their time indoors. Therefore, indoor air pollution could be a greater health hazard than outdoor air pollution. The possible components include tobacco smoke, the products of cooking, cleaning materials, insecticide sprays, and contaminants such as asbestos from work clothing. In the USA the Environmental Protection Agency has categorized indoor air pollution as one of the top five public health concerns. The subject receives close attention in Donald Wigle's Child Health and the Environment. Children are particularly vulnerable to indoor air pollutants because they inhale relatively high volumes of air per unit body weight and play on the floor where contaminant levels tend to be particularly high. The immaturity of organ systems and metabolic functions adds to the vulnerability of young children. Furthermore, children have little control over their own environment. The poor housing conditions of those from disadvantaged backgrounds may expose them to the pollutant hazards of dampness, degradation of building materials and lack of ventilation. Environmental tobacco smoke (ETS) is probably the best studied indoor air pollutant and represents an important preventable cause of childhood respiratory illness and death. Maternal exposure to ETS has been associated with increased risk of spontaneous abortion, preterm birth, low birthweight and intrauterine growth retardation. Childhood exposure to ETS has been associated with lung function deficits, exacerbation of asthma in preschool children and increased risk of lower respiratory tract infections. Also maternal and postnatal ETS exposure has been associated (weakly) with brain tumours, leukaemia and lymphomas. A meta-analysis of 39 studies of sudden infant death syndrome showed odds ratios of 2.1 (95% CI 1.8-2.4) for maternal smoking and 1.9 (1.6-2.4) for postnatal ETS. Carbon monoxide is another important indoor air pollutant to which children may be extra-susceptible on account of their high metabolic rates. The developing fetus is likewise at risk, not only because it has a higher oxygen demand but also because fetal haemoglobin has a high affinity for carbon monoxide. Volatile organic chemicals in the indoor environment include formaldehyde from chip-board, which can cause respiratory symptoms such as wheezing in children. Other such chemicals that may impact on child health include benzene from motor vehicles and paint solvents. In addition to chemicals, biological material from pets or house dust mites may be important. Pet allergens have been associated with asthma sensitization and house dust mites with asthma development. House dust mites and other allergens tend to be most concentrated on the floor, where children play. Child Health and the Environment, the first textbook of its kind, provides comprehensive coverage of current research areas. The early chapters give an overview of child environmental health, environmental epidemiology and risk assessment. The following nine chapters each address specific hazards including metals, polychlorinated biphenyls and dioxins, pesticides, hormonally active agents, radiation, indoor and outdoor air, and water. Each chapter is divided into health effects, exposures and risk management. The book does an excellent job at drawing together work from diverse areas and highlighting the many gaps in our knowledge.

Gaseous nitrogen oxides stimulate cell cycle progression by retinoblastoma phosphorylation via activation of cyclins/Cdks [correction

Nitrogen oxides (NOx) are important indoor and outdoor air pollutants. Many studies have indicated that NOx gas causes lung tissue damage by its oxidation properties and its free radicals. In a previous study we demonstrated that NOx gas induced proliferation of human lung fibroblast MRC-5 cells. In this study we show that NOx gas stimulates MRC-5 cell proliferation by retinoblastoma (Rb) phosphorylation via activation of cyclin-cell division protein kinase (cdk) complexes [correction]. Western blot and immunoprecipitation data showed that NOx gas increased the expressions of cyclinA/cdk2, cyclinD1/cdk4, and cyclinE/cdk2 complexes in the cells at 9 h after treatment. The levels of phospho-Rb were also increased and cdk inhibitors (CKIs) p27 and p16 were apparently decreased. These data suggested that NOx gas stimulates cell-cycle progression by Rb phosphorylation via activation of cyclin-cdk complexes and inhibition of CKIs. In conclusion, the NOx-gas that induced lung fibroblast cell proliferation by stimulation of cell-cycle progression may contribute to lung fibrosis by NOx pollutants.