Working with cattle slurry on farms: emission and dispersion of hydrogen sulfide gas during stirring.

Over the past 15 years, there have been numerous fatalities related to working with animal slurry. Working with cattle slurry releases toxic gases, in particular, hydrogen sulphide (H2S), which can cause acute central nervous system toxicity, breathing difficulties, and death if exposed to high concentrations. Real-time measurements of H2S gas were taken over distance and time, during the stirring of cattle slurry on farms. Gas was measured at eight slurry stores with differing typical configurations of indoor or outdoor stores and with or without slatted flooring. Highest H2S gas levels were measured from indoor stores under slatted floors, and generally at positions closest to the stirrer or the point of maximum stirring, with levels decreasing with distance from source. Most of the data indicate H2S gas levels increase very rapidly after stirring starts, and mostly decline to baseline levels within 30 min post start of stirring. There were, however, circumstances where gas levels remained high and only started to decline once the stirrer had stopped. H2S gas levels at all farms, at all positions measured were consistently below 10 ppm within 30 min of the stirrer being stopped. The current data highlight areas of the farm and ways of working that have the potential for workers and others to be at risk of exposure to toxic slurry gases. The area should be left to ventilate naturally for at least 30 min after the stirrer has been stopped before re-entering buildings. Influencing the design of stirring equipment and future slurry stores would likely reduce the risk of worker exposure to slurry gases.

Modeling the effect of temperature and relative humidity on exposure to SARS-CoV-2 in a mechanically ventilated room.

Computational fluid dynamics models have been developed to predict airborne exposure to the SARS-CoV-2 virus from a coughing person in a mechanically ventilated room. The models were run with three typical indoor air temperatures and relative humidities (RH). Quantile regression was used to indicate whether these have a statistically significant effect on the airborne exposure. Results suggest that evaporation is an important effect. Evaporation leads to respiratory particles, particularly those with initial diameters between 20 and 100 µm, remaining airborne for longer, traveling extended distances and carrying more viruses than expected from their final diameter. In a mechanically ventilated room, with all of the associated complex air movement and turbulence, increasing the RH may result in reduced airborne exposure. However, this effect may be so small that other factors, such as a small change in proximity to the infected person, could rapidly counter the effect. The effect of temperature on the exposure was more complex, with both positive and negative correlations. Therefore, within the range of conditions studied here, there is no clear guidance on how the temperature should be controlled to reduce exposure. The results highlight the importance of ventilation, face coverings and maintaining social distancing for reducing exposure.

Modeling and experimental study of dispersion and deposition of respiratory emissions with implications for disease transmission.

The ability to model the dispersion of pathogens in exhaled breath is important for characterizing transmission of the SARS-CoV-2 virus and other respiratory pathogens. A Computational Fluid Dynamics (CFD) model of droplet and aerosol emission during exhalations has been developed and for the first time compared directly with experimental data for the dispersion of respiratory and oral bacteria from ten subjects coughing, speaking, and singing in a small unventilated room. The modeled exhalations consist of a warm, humid, gaseous carrier flow and droplets represented by a discrete Lagrangian particle phase which incorporates saliva composition. The simulations and experiments both showed greater deposition of bacteria within 1 m of the subject, and the potential for a substantial number of bacteria to remain airborne, with no clear difference in airborne concentration of small bioaerosols (<10 µm diameter) between 1 and 2 m. The agreement between the model and the experimental data for bacterial deposition directly in front of the subjects was encouraging given the uncertainties in model input parameters and the inherent variability within and between subjects. The ability to predict airborne microbial dispersion and deposition gives confidence in the ability to model the consequences of an exhalation and hence the airborne transmission of respiratory pathogens such as SARS-CoV-2.

Analysis of factors affecting containment with extracted partial enclosures using computational fluid dynamics.

The Health and Safety Executive's (HSE's) COSHH Essentials (HSE, 2002, COSHH Essentials: easy steps to control chemicals HSG193. 2nd edn. ISBN 0 71762737 3. Available at http://www.coshh-essentials.org.uk. Accessed 30 October 2013) provides guidance on identifying the approaches required to control exposure to chemicals in the workplace. The control strategies proposed in COSHH Essentials are grouped into four control approaches: general ventilation, engineering control, containment, or to seek specialist advice. We report the use of experimental measurements and computational fluid dynamics (CFD) modelling to examine the performance of an engineering control approach and a containment control approach. The engineering control approach simulated was an extracted partial enclosure, based on the COSHH Essentials G200, for which simulations were compared with data from experiments. The containment approach simulated was of drum filling (in an extracted partial enclosure), based on the COSHH Essentials G305. The influence of the following factors on containment was examined: face velocity, size and location of face opening, and movement and ventilation flows. CFD predictions of the engineering control approach agreed well with the majority of the experimental measurements demonstrating confidence in the modelling approach used. The results show that the velocity distribution at the face of the enclosure is not uniform and the location and size of the opening are significant factors affecting the flow field and hence the containment performance. The simulations of drum filling show the effect on containment of the movement of a drum through the face of an enclosure. Analysis of containment performance, using a tracer, showed that containment was affected by the interaction between the ventilation flow direction and drum movement and spacing. Validated CFD simulations are shown to be a useful tool for gaining insight into the flows in control strategies for exposure control and to aid the interpretation of experimental measurements. The results support the assumption in COSHH Essentials that the use of 'containment' as a control approach is capable of achieving a 100-fold reduction in potential exposure. Novel CFD modelling techniques have been used to create controlled containment scenarios, improve understanding of the flow behaviour in the scenarios, and provide information that may aid future containment design.

Field experiments to assess approaches for spray drift incident investigation.

BACKGROUND: Spray trials were conducted to determine the variation in primary spray drift deposition between trials under very similar conditions, in order to assess the feasibility of developing a computational tool to aid post-event investigations of pesticide spray incidents. Pesticide deposition was examined by analysis of filter paper and vegetation samples. RESULTS: Considerable variation in the drift profile was found. The overall estimate of the spray drift decay term was -1.13 (95% confidence interval -1.02 to -1.24), with statistically significant differences between plots. Variation in the drift profile between neighbouring essentially identical plots indicates the variation in deposition that might be expected over small distances. Vegetation samples were found to have considerably lower capture efficiency than filter papers. Importantly, degradation of pesticides was found to have little effect on the pesticide drift profile over a 14 day period. CONCLUSIONS: The levels of spatial variation in spray drift deposits between runs and plots observed in this study suggest serious limitations to the inferences that may be drawn from limited numbers of post-incident samples. In particular, they would limit inferences about the spray conditions that could be drawn from an estimate of the drift profile derived from limited post-incident samples.