Bengt Ljungqvist

The Effect of Local Scavenging on Occupational Exposure to Nitrous Oxide

Despite evacuation of excess anaesthetic gases at the expiratory valve of the anaesthetic circuit and a general ventilation system producing 17–20 air changes per hour, mask anaesthesia often causes occupational exposure to anaesthetic gases exceeding the threshold limit values. The effect of a local air exhaust system, a local scavenger, on occupational exposure to nitrous oxide during paediatric mask anaesthesia was studied. The scavenger evacuated 140 m3 of air per hour and was placed at a distance of 20 cm from the face mask. In a very poorly ventilated operating theatre the exposure to nitrous oxide was reduced by 75% during the anaesthetic sessions and exposure to concentrations above 500 ppm was almost eliminated. The experiences from the installation and clinical use are discussed. Local scavenging is an excellent complement to the scavenging of excess gases at the expiratory valve, and it may be considered an alternative to expensive, high‐capacity ventilation systems.

Thermocamera, a Macroscopic Method for the Study of Pollution with Nitrous Oxide in Operating Theatres

Nitrous oxide (N2O) is used in high concentrations in inhalation anaesthesia and can serve as a tracer of other, more potent anaesthetic agents polluting the air of operating theatres. It has the quality of absorbing infra‐red light with a characteristic peak at 4.5 μm in the absorption spectrum. N2O in the operating‐room atmosphere will absorb infra‐red light emitted from a heat screen, and can therefore be registered by an infra‐red‐camera equipped with a filter eliminating waves outside the 4.5 μm waveband. The method was tested during paediatric inhalation anaesthesia. The infra‐red‐camera measurements are semi‐quantitative and sensitive to an extinction of about 1000 ppmcm, comparable to a N2O concentration of 100 ppm measured by an infra‐red N2O monitor. It was demonstrated that major pollution occurs during mask anaesthesia and after extubation. The polluting gas is insufficiently evacuated by the operating theatre ventilation. The method makes it possible to visualize the dispersion of spilled or leaking N2O, and is therefore of value when constructing and evaluating new scavenging equipment and in producing educational material.

A comparison between tracer gas and tracer particle techniques in evaluating the efficiency of ventilation in operating theatres

Operating theatres are ventilated for a number of reasons, one of them being to keep numbers of airborne bacteria low at the operation wound. No matter how air is brought into the room, bacteria are removed by dilution rather than by air currents, because of turbulence caused by heat liberated by people and equipment and by movement in the room (Lidwell & Williams, 1960). With ventilation rates up to 20 air changes/hour, the dilution may differ at different sites in the room depending on the design of its ventilation system.

Thermocamera Studies of Enflurane and Halothane Vapours

The ability of enflurane and halothane to absorb infra‐red (IR) energy was used to visualize their vapours; IR radiation was emitted by a heated screen and the absorption was studied with an IR camera. Even small concentrations of enflurane (0.2 vol%) and halothane (0.5 vol%) could be detected when released into the operating room atmosphere. Enflurane and nitrous oxide were dispersed in a similar way when they leaked from the face mask. Thus when anaesthetic pollution is monitored in the operating room, measurements of the concentration of nitrous oxide are sufficient for routine purposes. The IR method has added a new dimension to the study of occupational exposure to otherwise invisible gases.

10 – LOCAL VENTILATION

This chapter describes the aerodynamic principles, models, and equations that govern the flow and the contaminant presence and transport in a designated volume of a workroom. Local ventilation is often a very important part of the ventilation system, both in function and in construction. By using a local ventilation system of good design less air is needed to reach a specific contaminant level than is possible with general ventilation. Proper design and construction of a local ventilation system must account for hood flow rate, contaminant generation process and rate, and the generated flow rate of contaminated air. Thus, knowledge about airflow mechanics, process performance, and the contaminant source is essential. The purpose of local ventilation is to control the transport of contaminants at or near the source of emission, thus minimizing the contaminants in the workplace air. All local ventilation systems can, in principle, be manufactured for use in one or more of three different modes: fixed, flexible, and mobile.

SOME OBSERVATIONS ON AIR MOVEMENTS IN CLEAN ROOMS WITH UNIDIRECTIONAL AIR FLOW

ABSTRACT To avoid particle contamination in clean process production, unidirectional air flow with HEPA-filtered air is used, either in the entire room or partly in the critical process region (clean air zones), the purpose of this presentation is to describe a number of observations in unidirectional air flow and to discuss the interaction between air movements and the dispersion of airborne contaminants, it has been shown, by using smoke photography technique, that wake regions and vortex streets can easily be formed behind the working operator and objects. If a contaminant is emitted in the region of a vortex an accumulation can occur. It is important for the user to investigate that such vortices do not occur in the clean working zone.