Validation of krypton as a new tracer gas for the standardization tests of collective and individual protection systems.

Sulfur hexafluoride (SF6) is the reference tracer gas in many international standards for characterizing respiratory protective devices (RPD), fume cupboards, building ventilations, and other installations. However, due to its significant impact on global warming, its use is becoming increasingly restrictive. Krypton 84 (Kr) was chosen to be a possible replacement based on theoretical and practical criteria for the properties that a substitute gas should possess. While compliance with these criteria is generally sufficient to guarantee the reliability of the choice, it is essential in the case of widespread use such as a standard to validate experimentally that this tracer has the same behavior as SF6. In this regard, numerous tests have been carried out to characterize the face leakage of RPD and the rupture of containment of fume cupboards performance tests under different operating conditions. The results obtained are identical with both tracers and lead us to propose the use of Kr as a new reference gas in standards for which SF6 was used.

Tracking and Reducing SF6 Usage in Radiation Oncology: A Step Toward Net-Zero Health Care Emissions.

Sulfur hexafluoride (SF6) is a widely used insulating gas in medical linear accelerators (LINACs) due to its high dielectric strength, heat transfer capabilities, and chemical stability. However, its long lifespan and high Global Warming Potential (GWP) make it a significant contributor to the environmental impact of radiation oncology. SF6 has an atmospheric lifespan of 3200 years and a GWP 23,000 times that of carbon dioxide. The amount of SF6 that can be emitted through leakage from machines is also concerning. It is estimated that the approximate 15,042 LINACs globally may leak up to 64,884,185.9 carbon dioxide equivalent per year, which is the equivalent greenhouse gas emissions of 13,981 gasoline-powered passenger vehicles driven for 1 year. Despite being regulated as a greenhouse gas under the United Nations Framework Convention on Climate Change, SF6 use within health care is often exempt from regulation, and only a few states in the United States have specific SF6 management regulations. This article highlights the need for radiation oncology centers and LINAC manufacturers to take responsibility for minimizing SF6 emissions. Programs that track usage and disposal, conduct life-cycle assessments, and implement leakage detection can help identify SF6 sources and promote recovery and recycling. Manufacturers are investing in research and development to identify alternative gases, improve leak detection, and minimize SF6 gas leakage during operation and maintenance. Alternative gases with lower GWP, such as nitrogen, compressed air, and perfluoropropane, may be considered as replacements for SF6; however, more research is needed to evaluate their feasibility and performance in radiation oncology. The article emphasizes the need for all sectors, including health care, to reduce their emissions to meet the goals of the Paris Agreement and ensure the sustainability of health care and our patients. Although SF6 is practical in radiation oncology, its environmental impact and contribution to the climate crisis cannot be ignored. Radiation oncology centers and manufacturers must take responsibility for reducing SF6 emissions by implementing best practices and promoting research and development around alternatives. To meet global emissions reduction goals and protect both planetary and patient health, the reduction of SF6 emissions will be essential.

Sulfur hexafluoride in the marine atmosphere and surface seawater of the Western Pacific and Eastern Indian Ocean.

Sulfur hexafluoride (SF6) is a powerful greenhouse gas with a high global warming potential. While SF6 emissions from urban areas have been extensively studied, our knowledge about SF6 concentrations in the oceanic atmosphere and its air-sea exchange remains limited. Herein, the concentrations of SF6 in the atmosphere and surface seawater of the WPO (Western Pacific Ocean) and EIO (Eastern Indian Ocean) were comprehensively characterized from 2019 to 2022 in the first long-term study. The mean mixing ratios of SF6 over the WPO and EIO during 2019-2020 (2021-2022) were 10.9 (11.2) and 10.9 (11.1) ppt, respectively. The atmospheric SF6 concentration over the WPO and EIO increased at rates of 0.40 ± 0.06 and 0.58 ± 0.28 ppt yr-1, respectively, surpassing previously reported annual growth rates. The faster growth was primarily attributed to the influence of polluted air masses originating from eastern Asian countries, particularly Japan, Northeast China, and India. This might explain why the radiative forcing caused by SF6 in the study region was higher than the global average. The concentrations of SF6 in the surface seawater of the WPO and EIO ranged from 0.33 to 2.54 fmol kg-1, and the distribution was affected by atmospheric concentrations and ocean currents. Estimated air-sea fluxes revealed that the ocean acted as a significant sink of atmospheric SF6, and the preliminary estimation suggested oceanic uptake accounts for about 7% of annual global SF6 emissions. Based on these findings, we tentatively suggest that the strength of the ocean as a sink of SF6 may warrant reassessment. The global oceanic uptake of SF6 has the potential to reduce its global abundance and environmental impacts.

Transient tracer gas measurements: Development and evaluation of a fast-response SF6 measuring system based on quartz-enhanced photoacoustic spectroscopy.

This study aims to develop a fast-response sulfur hexafluoride (SF6 ) measuring system, and evaluate its performance in tracer gas measurements for studying transient airborne contaminant transport. The new system is based on a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor using a quantum cascade laser. Transient SF6 tracer gas measurements were carried out in an environmental chamber with an instantaneous source using both the QEPAS system and a traditional commercial instrument. Real-time SF6 concentrations, peak SF6 concentrations and average SF6 concentrations for one room time constant under two air change rates obtained by the two instruments were compared. The results show that the QEPAS system, which features a 0.4 s data acquisition interval, can provide detailed real-time SF6 concentrations even when the concentration is changing rapidly. The QEPAS system successfully captured the peak SF6 concentrations for all the studies cases, while commercial instrument failed in most studied cases. In most of the cases, the two instruments obtained similar average SF6 concentrations for one room time constant. However, when the concentration was in rapid change, the two systems would report significantly different results. The QEPAS system can be potentially applied in transient tracer gas measurements under complex scenarios.

Multiple breath washout (MBW) testing using sulfur hexafluoride: reference values and influence of anthropometric parameters.

BACKGROUND: Multiple breath washout (MBW) using sulfur hexafluoride (SF6) has the potential to reveal ventilation heterogeneity which is frequent in patients with obstructive lung disease and associated small airway dysfunction. However, reference data are scarce for this technique and mostly restricted to younger cohorts. We therefore set out to evaluate the influence of anthropometric parameters on SF6-MBW reference values in pulmonary healthy adults. METHODS: We evaluated cross-sectional data from 100 pulmonary healthy never-smokers and smokers (mean 51 (SD 20), range 20-88 years). Lung clearance index (LCI), acinar (Sacin) and conductive (Scond) ventilation heterogeneity were derived from triplicate SF6-MBW measurements. Global ventilation heterogeneity was calculated for the 2.5% (LCI2.5) and 5% (LCI5) stopping points. Upper limit of normal (ULN) was defined as the 95th percentile. RESULTS: Age was the only meaningful parameter influencing SF6-MBW parameters, explaining 47% (CI 33% to 59%) of the variance in LCI, 32% (CI 18% to 47%) in Sacin and 10% (CI 2% to 22%) in Scond. Mean LCI increases from 6.3 (ULN 7.4) to 8.8 (ULN 9.9) in subjects between 20 and 90 years. Smoking accounted for 2% (CI 0% to 8%) of the variability in LCI, 4% (CI 0% to 13%) in Sacin and 3% (CI 0% to 13%) in Scond. CONCLUSION: SF6-MBW outcome parameters showed an age-dependent increase from early adulthood to old age. The effect was most pronounced for global and acinar ventilation heterogeneity and smaller for conductive ventilation heterogeneity. No influence of height, weight and sex was seen. Reference values can now be provided for all important SF6-MBW outcome parameters over the whole age range. TRIAL REGISTRATION NUMBER: NCT04099225.

Analysis of Gaseous By-Products of CF3I and CF3I-CO2 after High Voltage Arcing Using a GCMS.

Increasing demand for an alternative insulation medium to sulphur hexafluoride (SF6) has led to the investigation of new environmentally friendly insulation gases which could be used in high voltage equipment on the electrical power network. One such alternative, which is currently being explored by researchers, is Trifluoroiodomethane (CF3I) which could potentially be used in a gas mixture with carbon dioxide (CO2) as an insulation medium. In this paper an analysis of gaseous by-products detected as a result of high voltage breakdown through pure CF3I and a CF3I-CO2 gas mixture across a sphere-sphere electrode arrangement is given. Gas chromatography and mass spectrometry (GCMS) is used to identify the gaseous by-products produced as a result of high voltage arcing which causes the gas between the electrodes to dissociate. Analysing these gas by-products helps to identify the long-term behaviour of the gas mixture in high voltage equipment.

Comparison of lung clearance index determined by washout of N2 and SF6 in infants and preschool children with cystic fibrosis.

BACKGROUND: Multiple-breath washout (MBW) has been shown to detect early impairment of lung function in children with cystic fibrosis (CF). Nitrogen (N2) or sulfur hexafluoride (SF6) can be used as tracer gas for MBW. Recent data indicated higher lung clearance index (LCI) values measured with N2-MBW than concurrent SF6-MBW in older children and adults, however, a comparison in infants and younger children, as well as to other outcome measures of CF lung disease is pending. METHODS: N2- and SF6-MBW were performed consecutively in 31 sedated infants and preschool children with CF (mean age, 2.3 ±â€¯0.8 years) and 20 controls (mean age, 2.3 ±â€¯1.1 years) using the Exhalyzer D system. Children with CF also underwent chest magnetic resonance imaging (MRI). RESULTS: Mean difference (95% CI) in LCI between N2- and SF6-MBW was 1.1 ±â€¯0.4 (0.9 to 1.3) in controls and 2.1 ±â€¯1.9 (1.4 to 2.8) in CF. Agreement between N2- and SF6-LCI was poor in children with CF. N2-LCI and SF6-LCI correlated with MRI, however N2-LCI showed a higher concordance with MRI than SF6-LCI. The absolute difference between N2- and SF6-LCI values increased with the severity of CF lung disease as determined by MRI scores. CONCLUSION: N2-LCI values were higher than SF6-LCI values in infants and preschool children with CF and controls. Better concordance of N2-LCI than SF6-LCI with chest MRI scores point towards of a higher sensitivity of N2-LCI to detect early lung disease in children with CF.

A Synoptic View of the Ventilation and Circulation of Antarctic Bottom Water from Chlorofluorocarbons and Natural Tracers.

Antarctic Bottom Water (AABW) is the coldest, densest, most prolific water mass in the global ocean. AABW forms at several distinct regions along the Antarctic coast and feeds into the bottom limb of the meridional overturning circulation, filling most of the global deep ocean. AABW has warmed, freshened, and declined in volume around the globe in recent decades, which has implications for the global heat and sea level rise budgets. Over the past three decades, the use of tracers, especially time-varying tracers such as chlorofluorocarbons, has been essential to our understanding of the formation, circulation, and variability of AABW. Here, we review three decades of temperature, salinity, and tracer data and analysis that have led to our current knowledge of AABW and how the southern component of deep-ocean ventilation is changing with time.

Density-driven transport of gas phase chemicals in unsaturated soils.

Variations of gas phase density are responsible for advective and diffusive transports of organic vapors in unsaturated soils. Laboratory experiments were conducted to explore dense gas transport (sulfur hexafluoride, SF6) from different source densities through a nitrogen gas-dry soil column. Gas pressures and SF6 densities at transient state were measured along the soil column for three transport configurations (horizontal, vertically upward and vertically downward transport). These measurements and others reported in the literature were compared with simulation results obtained from two models based on different diffusion approaches: the dusty gas model (DGM) equations and a Fickian-type molar fraction-based diffusion expression. The results show that the DGM and Fickian-based models predicted similar dense gas density profiles which matched the measured data well for horizontal transport of dense gas at low to high source densities, despite the pressure variations predicted in the soil column were opposite to the measurements. The pressure evolutions predicted by both models were in trend similar to the measured ones for vertical transport of dense gas. However, differences between the dense gas densities predicted by the DGM and Fickian-based models were discernible for vertically upward transport of dense gas even at low source densities, as the DGM-based predictions matched the measured data better than the Fickian results did. For vertically downward transport, the dense gas densities predicted by both models were not greatly different from our experimental measurements, but substantially greater than the observations obtained from the literature, especially at high source densities. Further research will be necessary for exploring factors affecting downward transport of dense gas in soil columns. Use of the measured data to compute flux components of SF6 showed that the magnitudes of diffusive flux component based on the Fickian-type diffusion expressions in terms of molar concentration, molar fraction and mass density fraction gradient were almost the same. However, they were greater than the result computed with the mass fraction gradient for >24% and the DGM-based result for more than one time. As a consequence, the DGM-based total flux of SF6 was in magnitude greatly less than the Fickian result not only for horizontal transport (diffusion-dominating) but also for vertical transport (advection and diffusion) of dense gas. Particularly, the Fickian-based total flux was more than two times in magnitude as much as the DGM result for vertically upward transport of dense gas.

Short communication: Comparison of the GreenFeed system with the sulfur hexafluoride tracer technique for measuring enteric methane emissions from dairy cows.

The objective of this study was to compare 2 commonly used techniques for measuring methane emissions from ruminant animals: the GreenFeed (GF) system and the sulfur hexafluoride (SF6) technique. The study was part of a larger experiment in which a methane inhibitor, 3-nitrooxypropanol, fed at 4 application rates (0, 40, 60, and 80 mg/kg of feed dry matter) decreased enteric methane emission by an average of 30% (measured by both GF and SF6) in a 12-wk experiment with 48 lactating Holstein cows fed a total mixed ration. The larger experiment used a randomized block design and was conducted in 2 phases (February to May, phase 1, and June to August, phase 2), with 2 sets of 24 cows in each phase. Using both GF and SF6 techniques, methane emission data were collected simultaneously during experimental wk 2, 6, and 12 (phase 1) and 2, 9, and 12 (phase 2), which corresponded to a total of 6 sampling periods. During each sampling period, 8 spot samples of gas emissions (staggered over a 3-d period) were collected from each cow using GF, as well as 3×24-h collections using the SF6 technique. Methane emission data were averaged per cow for the statistical analysis. The mean methane emission was 373 (standard deviation=96.3) and 405 (standard deviation=156) g/cow per day for GF and SF6, respectively. Coefficients of variation for the 2 methods were 25.8 and 38.6%, respectively; correlation and concordance between the 2 methods were 0.40 and 0.34, respectively. The difference in methane emission between the 2 methods (SF6 - GF) within treatment was from 46 to 144 and 24 to 27 g/d for phases 1 and 2, respectively. In the conditions of this experiment, the SF6 technique produced larger variability in methane emissions than the GF method. The overall difference between the 2 methods was on average about 8%, but was not consistent over time, likely influenced by barn ventilation and background methane and SF6 concentrations.

Quantifying capture efficiency of gas collection wells with gas tracers.

A new in situ method for directly measuring the gas collection efficiency in the region around a gas extraction well was developed. Thirteen tests were conducted by injecting a small volume of gas tracer sequentially at different locations in the landfill cell, and the gas tracer mass collected from each test was used to assess the collection efficiency at each injection point. For 11 tests the gas collection was excellent, always exceeding 70% with seven tests showing a collection efficiency exceeding 90%. For one test the gas collection efficiency was 8±6%. Here, the poor efficiency was associated with a water-laden refuse or remnant daily cover soil located between the point of tracer injection and the extraction well. The utility of in situ gas tracer tests for quantifying landfill gas capture at particular locations within a landfill cell was demonstrated. While there are certainly limitations to this technology, this method may be a valuable tool to help answer questions related to landfill gas collection efficiency and gas flow within landfills. Quantitative data from tracer tests may help assess the utility and cost-effectiveness of alternative cover systems, well designs and landfill gas collection management practices.

Short communication: Use of a portable, automated, open-circuit gas quantification system and the sulfur hexafluoride tracer technique for measuring enteric methane emissions in Holstein cows fed ad libitum or restricted.

The objective of this study was to measure enteric CH4 emissions using a new portable automated open-circuit gas quantification system (GQS) and the sulfur hexafluoride tracer technique (SF6) in midlactation Holstein cows housed in a tiestall barn. Sixteen cows averaging 176 ± 34 d in milk, 40.7 ± 6.1 kg of milk yield, and 685 ± 49 kg of body weight were randomly assigned to 1 out of 2 treatments according to a crossover design. Treatments were (1) ad libitum (adjusted daily to yield 10% orts) and (2) restricted feed intake [set to restrict feed by 10% of baseline dry matter intake (DMI)]. Each experimental period lasted 22d, with 14 d for treatment adaptation and 8d for data and sample collection. A common diet was fed to the cows as a total mixed ration and contained 40.4% corn silage, 11.2% grass-legume haylage, and 48.4% concentrate on a dry matter basis. Spot 5-min measurements using the GQS were taken twice daily with a 12-h interval between sampling and sampling times advanced 2h daily to account for diurnal variation in CH4 emissions. Canisters for the SF6 method were sampled twice daily before milking with 4 local background gas canisters inside the barn analyzed for background gas concentrations. Enteric CH4 emissions were not affected by treatments and averaged 472 and 458 g/d (standard error of the mean = 18 g/d) for ad libitum and restricted intake treatments, respectively (data not shown). The GQS appears to be a reliable method because of the relatively low coefficients of variation (ranging from 14.1 to 22.4%) for CH4 emissions and a moderate relationship (coefficient of determination = 0.42) between CH4 emissions and DMI. The SF6 resulted in large coefficients of variation (ranging from 16.0 to 111%) for CH4 emissions and a poor relationship (coefficient of determination = 0.17) between CH4 emissions and DMI, likely because of limited barn ventilation and high background gas concentration. Research with improved barn ventilation systems or outdoors is warranted to further assess the GQS and SF6 methodologies.

An Ultra-Trace Analysis Technique for SF6 Using Gas Chromatography with Negative Ion Chemical Ionization Mass Spectrometry.

Sulfur hexafluoride (SF6) is widely used as a tracer gas because of its detectability at low concentrations. This attribute of SF6 allows the quantification of both small-scale flows, such as leakage, and large-scale flows, such as atmospheric currents. SF6's high detection sensitivity also facilitates greater usage efficiency and lower operating cost for tracer deployments by reducing quantity requirements. The detectability of SF6 is produced by its high molecular electronegativity. This property provides a high potential for negative ion formation through electron capture thus naturally translating to selective detection using negative ion chemical ionization mass spectrometry (NCI-MS). This paper investigates the potential of using gas chromatography (GC) with NCI-MS for the detection of SF6. The experimental parameters for an ultra-trace SF6 detection method utilizing minimal customizations of the analytical instrument are detailed. A method for the detection of parts per trillion (ppt) level concentrations of SF6 for the purpose of underground ventilation tracer gas analysis was successfully developed in this study. The method utilized a Shimadzu gas chromatography with negative ion chemical ionization mass spectrometry system equipped with an Agilent J&W HP-porous layer open tubular column coated with an alumina oxide (Al2O3) S column. The method detection limit (MDL) analysis as defined by the Environmental Protection Agency of the tracer data showed the method MDL to be 5.2 ppt.

Flow characteristics and spillage mechanisms of an inclined quad-vortex range hood subject to influence from draft.

The flow and spillage characteristics of an inclined quad-vortex (IQV) range hood subject to the influence of drafts from various directions were studied. The laser-assisted smoke flow visualization technique was used to reveal the flow characteristics, and the tracer-gas (sulfur hexafluoride) concentration detection method was used to indicate the quantitative values of the capture efficiency of the hood. It was found that the leakage mechanisms of the IQV range hood are closely related to the flow characteristics. A critical draft velocity of about 0.5 m/s and a critical face velocity of about 0.25 m/s for the IQV range hood were found. When the IQV range hood was influenced by a draft with a velocity larger than the critical draft velocity, the spillage of pollutants became significant and the pollutant spillage rate increased with increasing draft velocity. At draft velocities less than or equal to the critical value, no containment leakages induced by the turbulence diffusion, reverse flow, or boundary-layer separation were observed, and the capture efficiency was about 100%. The IQV range hood exhibited a high ability to resist the influences of lateral and frontal drafts. The capture efficiency of the IQV range hood operated at the suction flow rate 5 to 9 m(3)/min is higher than that of the conventional range hood operated at 11 to 15 m(3)/min.

High performance simulation of environmental tracers in heterogeneous domains.

In this study, we use PFLOTRAN, a highly scalable, parallel, flow, and reactive transport code to simulate the concentrations of 3H, 3He, CFC-11, CFC-12, CFC-113, SF6, 39Ar, and the mean groundwater age in heterogeneous fields on grids with an excess of 10 million nodes. We utilize this computational platform to simulate the concentration of multiple tracers in high-resolution, heterogeneous 2D and 3D domains, and calculate tracer-derived ages. Tracer-derived ages show systematic biases toward younger ages when the groundwater age distribution contains water older than the maximum tracer age. The deviation of the tracer-derived age distribution from the true groundwater age distribution increases with increasing heterogeneity of the system. However, the effect of heterogeneity is diminished as the mean travel time gets closer to the tracer age limit. Age distributions in 3D domains differ significantly from 2D domains. 3D simulations show decreased mean age, and less variance in age distribution for identical heterogeneity statistics. High-performance computing allows for investigation of tracer and groundwater age systematics in high-resolution domains, providing a platform for understanding and utilizing environmental tracer and groundwater age information in heterogeneous 3D systems.

Limitations of the use of environmental tracers to infer groundwater age.

Apparent ages obtained from the measured concentrations of environmental tracers have the potential to inform recharge rates, flow rates, and assist in the calibration of groundwater models. A number of studies have investigated sources of error in the relationships between the apparent ages, and the age assumed by models to relate this quantity to an aquifer property (e.g., recharge). These studies have also provided a number of techniques for correcting the known biases of apparent ages. In this paper, we review some of the concepts of age bias. We then demonstrate this bias through the use on four numerical examples, and assess the accuracy of correction methods in overcoming this bias. We examine this for CFCs, SF6, 3H/3He, 39Ar, and 14C. We demonstrate that in our four simple steady-state aquifer examples, bias occurs for a wide range of environmental tracers and flow configurations. When applying correction methods, we found that the values obtained are limited by the model assumptions. Models accounting for exchange with aquitards represent whole mobile zones and not discrete well screens. Mean transit times (comparable to mean ages) obtained from lumped parameter models deviate from actual values as the assumed distribution varies from the actual distribution. Methods that use multiple tracer ages are limited to ranges where both tracers report apparent ages. Our findings suggest that the incorporation of environmental tracer data into the understanding of groundwater systems requires approaches such as the direct use of concentrations, or the simulation of full age distributions.

Atmospheric sulfur hexafluoride in-situ measurements at the Shangdianzi regional background station in China.

We present in-situ measurements of atmospheric sulfur hexafluoride (SF6) conducted by an automated gas chromatograph-electron capture detector system and a gas chromatography/mass spectrometry system at a regional background site, Shangdianzi, in China, from June 2009 to May 2011, using the System for Observation of Greenhouse gases in Europe and Asia and Advanced Global Atmospheric Gases Experiment (AGAGE) techniques. The mean background and polluted mixing ratios for SF6 during the study period were 7.22 × 10⁻¹² (mol/mol, hereinafter) and 8.66 × 10⁻¹², respectively. The averaged SF6 background mixing ratios at Shangdianzi were consistent with those obtained at other AGAGE stations located at similar latitudes (Trinidad Head and Mace Head), but larger than AGAGE stations in the Southern Hemisphere (Cape Grim and Cape Matatula). SF6 background mixing ratios increased rapidly during our study period, with a positive growth rate at 0.30 × 10⁻¹² year⁻¹. The peak to peak amplitude of the seasonal cycle for SF6 background conditions was 0.07 × 10⁻¹², while the seasonal fluctuation of polluted conditions was 2.16 × 10⁻¹². During the study period, peak values of SF6 mixing ratios occurred in autumn when local surface horizontal winds originated from W/WSW/SW/SWS/S sectors, while lower levels of SF6 mixing ratios appeared as winds originated from N/NNE/NE/ENE/E sectors.

Evaluation of the SF6 tracer technique for estimating methane emission rates with reference to dairy cows using a mechanistic model.

A dynamic, mechanistic model of the sulfur hexafluoride (SF6) tracer technique, used for estimating methane (CH4) emission rates from ruminants, was constructed to evaluate the accuracy of the technique. The model consists of six state variables and six zero-pools representing the quantities of SF6 and CH4 in rumen and hindgut fluid, in rumen and hindgut headspace, and in blood and collection canister. The model simulates flows of CH4 and SF6 through the body, subsequent eructation and exhalation and accumulation in a collection canister. The model predicts CH4 emission by multiplying the SF6 release rate of a permeation device in the rumen by the ratio of CH4:SF6 in collected air. This prediction is compared with the actual CH4 production rate, assumed to be continuous and used as a driving variable in the model. A sensitivity analysis was conducted to evaluate the effect of changes in several parameters. The predicted CH4 emission appeared sensitive to parameters affected by the difference in CH4:SF6 ratio in exhaled and eructed air respectively, viz., hindgut fractional passage rate and hindgut CH4 production. This is caused by the difference in solubility of CH4 and SF6 and by hindgut CH4 production. In addition, the predicted CH4 emission rate appeared sensitive to factors that affect proportions of exhaled and eructed air sampled, i.e., eructation time fraction, exhalation time fraction, and distance from sampling point to mouth/nostrils. Changes in rumen fractional passage rate, eructation rate, SF6 release rate, background values and air sampling rate did not noticeably affect the predicted CH4 emission. Simulations with (13)CH4 as an alternative tracer show that the differences and sensitivity to parameters greatly disappear. The model is considered a useful tool to evaluate critical points in the SF6 technique. Data from in vivo experiments are needed to further evaluate model simulations.