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January 9, 2013

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Protective workwear – Chemical world

Fig 2: Results for liquid toluene permeating through a CPC fabric into a flowing nitrogen gas stream, illustrated by a plot of permeation flux versus exposure time. The horizontal dashed line shows the USA standard threshold flux of 0.1µg cm-2 min-1, which, from the intersections with the measured flux plots, give breakthrough times in the range 19-30 minutes. The corresponding European threshold flux of 1.0µg cm-2 min-1 is never reached in this example.

Following first contact of the toluene with the fabric, there is a lag when the flux is initially zero, as it takes time for toluene to enter from the donor compartment, diffuse across the fabric, and exit to the receiver compartment. The flux then rises to reach a plateau value of about 0.25–0.45g cm-2 min-1. The flux will remain at this steady level until the toluene in the donor compartment depletes entirely.

Figure 2 shows results for a control measurement with no added toluene and three repeat measurements. In general, the precision of individual runs and data points is determined by the detector and produces an uncertainty of less than 2 per cent. The variation between runs mainly originates from area-to-area variations in fabric thickness and is typical of all commercially-available polymer films, owing to the nature of manufacturing processes.

Measurements of permeation flux versus time are used to extract breakthrough times. Currently, European performance standards for chemical protective clothing permeation (for example, EN 943-1 and EN 14605) rely on a normalised breakthrough time (NBT). This is the time from first contact of the challenge chemical with the fabric before the flux reaches a specified value. In Europe, the specified flux is 1.0µg cm-2 min-1. By contrast, the US standard (ASTM F739) specifies the value as 0.1µg cm-2 min-1.1

Using measurements from independent, accredited laboratories, manufacturers of chemical protective clothing classify the protection of their CPC fabrics against individual challenge chemicals as classes 1-6, according to the “performance classifications” below.

Class    Breakthrough time
6           >480 min
5           >240 min
4           >120 min
3           >60 min
2           >30 min
1           >10 min

Table 1: Performance classification table for chemical-permeation breakthrough times (from EN 14325:2004)

The problem with this approach is that performance classification based on NBTs is insufficient to fully assess the usage of chemical protective clothing in real-world applications. For example, it does not take account of the influences of temperature, pressure (e.g. kneeling in a pool of liquid), flexing, etc. More importantly, it does not necessarily provide an accurate indication of “safe wear time” with respect to a given chemical, which requires real-life exposure scenarios to be considered. Risks may range from a small liquid splash on a tiny area of the suit, to saturation of the whole suit, or exposure to vapour over a long period.2

In addition, the fact that different chemicals can have very different levels of risk, owing to their widely varying intrinsic toxicities, needs to be taken into account. Hence, there is a need to relate CPC safety standards to more widely-used frameworks for the assessment and specification of the toxicity levels and safe exposure limits of chemicals.

Relating chemical permeation to “safe wear time”
In the UK, the HSE provides land-use planning advice in terms of a specified level of toxicity (SLOT) defined by a set of descriptors, as follows:

  •     severe distress to almost everyone in the area;
  •     substantial fraction of exposed population requiring medical treatment;
  •     some people seriously injured, requiring prolonged treatment; and
  •     highly-susceptible people possibly being killed.

The SLOT dangerous toxic load (SLOT DTL – i.e. the quantity necessary for the conditions of SLOT to be met) for a particular chemical in the air is determined by two factors – the concentration (c) and the exposure time (t).

For many chemicals, the toxic load to produce a particular SLOT is simply the product of c multiplied by t.3 For example, the SLOT DTL for methyl isocyanate is 750 ppm.min; this means that the specified level of toxicity could be produced by, say, either exposure for five minutes to a concentration of 150 ppm, or by exposure to 6 ppm for 125 minutes (e.g. x ppm × y min = z ppm.min). This concept takes account of the total amount of the chemical absorbed during exposure and enables safety guidance to be soundly based on toxicity data for individual chemicals.

Let’s relate this method to chemical protective clothing performance. Permeation results provide measurements of the amount of a chemical accumulating inside the clothing over an exposure time. This data could therefore be used to estimate the increasing toxic load experienced by the CPC wearer over time. The “safe wear time” could then be estimated by the requirement that the toxic load must be less than the SLOT DTL value for the challenge chemical.

A better approach to assessing risk?
There are issues, however, in assessing toxic load resulting from chemical permeation through protective clothing.

The total amount of a chemical that ends up inside the suit after a certain exposure time is equal to the product of the permeation flux multiplied by surface area, over time. The first issue relates to estimating this surface area, which refers to the area of contact between the chemical and exterior surface of the suit and will depend on various circumstances. For instance, if the suit is fully immersed in the chemical (e.g. for a vapour), the surface area will be slightly larger than the average adult body surface area of 1.7m2.4 Clearly, the surface area will be much less if the suit is exposed only to some adhering splashes of a liquid chemical.

The second issue concerns the difficulty of relating SLOT DTL values to what might happen when an amount of chemical has accumulated inside a suit. For chemicals that vaporise into the air space inside the suit, the concentration (and the subsequent toxic load) can be estimated from the permeated amount and the volume of the inner-suit air space (about 5 litres). But  estimating toxic loads and their relation to SLOT DTL values for chemicals that do not vaporise inside chemical protective clothing remains problematic.

The approach can be illustrated by an example. Consider a chemical that permeates a protective suit with a steady flux of 0.05µg cm-2 min-1. This is less than the European standard threshold value of 1.0µg cm-2 min-1, so the breakthrough time would be recorded as more than 480 minutes under the current specification system (see table 1).

Let’s suppose that this chemical makes contact with the clothing over an area of 100cm2 for 100 minutes. Over this period, the total amount of chemical that accumulates inside is 500µg:
0.05µg cm-2 min-1 x 100cm2 x 100min = 500µg

If we assume that the permeated chemical evaporates into the air space inside the suit (of volume 5 litres and mass 6g), then the maximum concentration of the chemical in the air inside the suit is 80ppm:
500µg÷6g = 500µg÷6000000µg = 83.3µg÷1000000µg (approximately 80mg kg-1 (ppm))

Over the 100-minute exposure time, the inside-suit concentration ramps up from 0 to 80ppm, so the toxic load experienced is approximately 4000ppm.min:
1/2 (100min ≈ 80ppm) = 4000ppm.min

This value exceeds the SLOT DTL of many toxic chemicals and so could be hazardous, even though this potential hazard is not indicated by the breakthrough time. This example shows that chemical protective clothing permeation standards based on the total amount of challenge chemical that enters the CPC over time can, in certain cases, provide better risk assessment than current standards based on permeation flux and breakthrough times. 

The European and International permeation test method, EN ISO 6529, describes a technique for detecting the mass of chemical permeating CPC over a given time, known simply as “cumulative permeation”. In 2007, the International Organisation for Standardisation (ISO) published its own CPC performance standard (ISO 16602),5 which specifies cumulative permeation data, and not NBT, to classify the performance of Types 1, 2, 3 and 4 CPC. However, this classification uses generic cumulative-permeation threshold values for all chemicals instead of incorporating chemical-specific values based on the variable toxicities of individual chemicals.

Conclusion
The barrier properties of chemical protective clothing to hazardous chemicals are primarily measured by determining how the permeation flux of the chemical through the material varies with exposure time.

According to current European standards, CPC performance classes are based on the breakthrough times required to reach a threshold flux. Although this system has the great advantage of simplicity, an improved risk assessment could be obtained by using the cumulative permeating mass that enters the CPC and the resulting toxic load, relative to specified limits of toxicity for individual chemicals. 

This issue is currently under debate by European technical committee working groups for the standardisation of both glove and chemical protective clothing performance standards. A joint working group has also been established, comprising protective glove, footwear and clothing experts, with the sole purpose of reviewing the current European test methods for permeation of chemicals.

Ultimately, we believe that enhanced worker protection would be delivered via the future adoption of cumulative permeation to replace, or be used in addition to, breakthrough time for classifying the permeation performance of chemical protective clothing.

At-a-glance glossary

  • CPC – Chemical protective clothing
  • EN 14325 – European Norm 14325: Protective clothing against chemicals. Test methods and performance     classification of chemical protective clothing materials, seams, joins and assemblages
  • Flux – Rate of substance flow through a barrier, given in mass per unit area per unit time    
  • ISO 16602 – International Standards Organisation 16602 (Protective clothing for protection against chemicals – Classification, labelling and performance requirements)
  • NBT – Normalised Breakthrough Time
  • SLOT – Specified Level of Toxicity


References

  1. These values are somewhat arbitrary and were primarily based on equipment detection capabilities at the time the first CPC test methods and performance standards were written. The facts that these numbers mean nothing by themselves other than being attributed to generic fabric performance, and that they differ by a factor of ten between Europe and the USA, are of little help and can cause confusion – underlining the need for new test methods and standards for chemical permeation 
  2. Some European norms and technical guidance documents have attempted to give further advice on these matters. For example, in PD CEN/TR 15419 (Protective clothing. Guidelines for selection, use, care and maintenance of chemical protective clothing), it states: “Although class 6 performance is to be preferred, fabrics that only achieve class 2 or 3 may still give adequate protection, provided that any surface contamination is washed off the garment promptly and that no gross chemical degradation is apparent.”
  3. Compilations of SLOT DTL values for many different chemicals are publicly available at www.hse.gov.uk/chemicals/haztox.htm
  4. Current, JD (1998): ‘A linear equation for estimating the body surface area in infants and children’, in Internet Journal of Anesthesiology, vol.2, no.2
  5. www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=45985

 

About the authors
Paul Bryce is technical director and Dr Michael Draper is technical services manager for Microgard Limited; Paul Fletcher is Professor of Physical Chemistry at the University of Hull.

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