THINK DETAILS FOR AIR TOXICITY

       Monitoring and body protection

        Gas monitoring by portable instruments

            Introduction

Purpose of monitoring

In chemical accidents it is crucial to monitor the air for concentrations of hazardous substances. The aim of gas monitoring is to assess both toxic and fire/explosion hazards as well as map the area where unprotected personnel should be evacuated and judge the appropriate level of body protection for response personnel. On some occasions the read-out values can be used directly for designing risk areas (cf. Figure 3 - 1). In other circumstances the instruments can be used to check risk areas that are already assessed or defined according to some calculation method or forecasting model.

The most difficult measuring task on a site of a chemical accident is to make rapid identification of unknown airborne substances by means of portable instruments. Such work requires sophisticated instruments and trained expertise often not available when the first risk assessment is going to be made. Proper use of portable gas monitoring instruments generally require exact knowledge of the gas identity. Luckily this is often the case. Especially when a single chemical substance is involved, its identity is often known to the responders.

Monitoring instruments
In the world market there are hundreds of manufacturers offering thousands of different models of hand-held, portable, transportable or mobile gas detection and monitoring instruments that can be used during chemical accidents for risk assessment and evaluation. It is a rapidly developing market and it is difficult to give good advice on particular brands and models of instruments. Therefore this Chapter is limited to rather brief discussions on essentials in this field. Ref. 51 is an example of a broad overview of principles of hazardous gas monitors and serves as a guide to the selection of types of instruments.

Calibration
Many instruments require calibration before use! It is therefore crucial to follow the instructions carefully in this respect.
 

Trace gas monitoring

The main objectives of trace gas monitoring in case of a chemical accident are to find dangerous locations of a toxic gas contaminated area and assess the outer limits where it is reasonably safe for unprotected personnel. This type of instruments must be able to detect very low concentrations of hazardous gases (ppm-levels).

Examples of types of hand-held or mobile trace gas detection and monitoring devices:
 

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Gas detection tubes

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IR trace gas detectors

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Semiconductor instruments

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Portable gas chromatographs

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Photoionization instruments

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Mobile mass spectrometers

Gas detector tubes, semiconductor instruments and some photoionization instruments are relatively simple hand-held devices. Note that these instrument cannot be used for accurate readings. The results are approximate. The monitoring should be performed according to section ”Performance” below.

Portable gas chromatographs and mobile mass spectrometers are examples of more sophisticated instruments. These instruments are miniaturized and automatized laboratory equipment. They generally give rather accurate readings but require skilled personnel to handle.
 

Performance
Monitoring should be performed by personnel, equipped with breathing apparatus, trained in monitoring instruments and familiar with their functioning.

The measurements should be carried out from outside (gas-free area) inwards to the dangerous area. The position, where the first recording is noticed, is the fringe of the evacuation area. By doing further monitoring around the place of the accident, a map of the evacuation area can be drawn.

However, it should be emphasized that a gas cloud in the reality often doesn’t give the uniform and smooth curves as shown in Figure 3 - 1. The boundary line, however, should be levelled along the outermost gas registrations.

Gas clouds often move irregularly due to the air turbulence and other environmental conditions.

If no monitoring device is available, a safe evacuation area must be established around the scene of the accident, initially with a great margin of safety until more information can support a change of the judgement.
 

Oxygen-Deficient Air Monitoring

The oxygen level in confined spaces, such as cargo holds or tanks, can decrease because of work being done, such as welding, cutting, or brazing. It can also be decreased by oxygen-consuming reactions (metal rusting or cargo oxidation) or through microbial action (fermentation).

The oxygen concentration is also decreased if air is displaced by another gas, such as inert gas, carbon dioxide, nitrogen or hydrocarbons. If such a gas causes total displacement of oxygen, an unprotected person will rapidly become unconscious and die.

Oxygen-deficient means that there is not enough oxygen in the space to safety breathe. Normal fresh air contains 20.8 percent oxygen compared to less than 19.5 percent in an oxygen-deficient atmosphere. Air that has less than 10 percent oxygen can rapidly cause unconsciousness and levels below 8 percent can quickly cause death.

The objective of oxygen-deficient air monitoring instruments is to assess the outer limits of an oxygen deficient area where it is reasonably safe for unprotected personnel (Oxygen concentration above 19.5%). Any atmosphere with less than 19.5% oxygen should not be entered without an approved self-contained breathing apparatus (SCBA).

Examples of monitoring devices are chemical celloxygen meters.