Occupational Health - Case Study

Crystal Glass Industry - Monitoring lead(Pb) in the work place

Summary of findings

The results conclude that a significant occupational health issue exists at crystal glass factories relating to ultra-fine lead particles (diameter less than 0.2 micrometer).

The consequences of not monitoring ultra-fine lead particles in the air at work stations of crystal glass factories are that:

  1. The likely threat to workers will remain unquantified and any abatement processes unmonitored.
  2. Assessments of the level of lead in the air from only larger particles may overestimate the risk to workers.

Scope

The aim of this work was to establish relationships between lead in the atmosphere at working places and lead contained in the blood of workers. Particularly, it is important to establish the fraction of the particular matter that is responsible for the majority of lead intake. WRAS systems with Humidity Control. were employed to obtain lead particle size distributions over the entire aerosol size range at several crystal glass factories (in the UK and Ireland). Humidity inside the sampling units was maintained at 85% Rh to match human lung conditions.

Lead particle size distributions were obtained in the size range from 2 nm to 20 mm (diameter).

Atomic Adsorption spectrometry was employed to quantify the amount of lead collected in different size sections. The flow rate was 20 l/min.

The main determinant of the behaviour of an atmospheric particle is its size. This is usually expressed in terms of the 'aerodynamic diameter', which refers to unit density spherical particles with the same aerodynamic properties, such as falling speed. In practice, except for very dense materials and clusters, the aerodynamic diameter is very similar to the geometric diameter as might be measured with a light microscope or electron microscope.

Measurements of particle size distributions of atmospheric aerosols usually show up to three groups of particles (or modes):

  • Nucleation mode: < 0.2 micrometer diameter. These are particles recently emitted from processes involving condensation of hot vapours (e.g. furnaces, smelters) or particles freshly formed within the atmosphere by gas to particle conversion (e.g. sulphuric acid particles from sulphur dioxide oxidation).
  • Accumulation mode: 0.2 - 2 micrometer diameter. This mode comprises particles, which have grown from the nucleation mode by coagulation or condensation of vapours.
  • Coarse mode: > 2 micrommeter diameter. These particles are mainly formed by mechanical attrition processes, and hence soil dust, sea spray and many industrial dusts fall within this mode.

A validated kinetic model of lead metabolism in humans (ICRP, 1975 and Leggett, 1993) has been employed to calculate lead in the blood of workers. Calculations have been based on the concept of the critical particle size, Dc. It was assumed that particles of smaller diameters than Dc could be dissolved in the lung fluid but larger particles (D > Dc) would not be dissolved and would be removed by the lung's self-cleaning mechanisms.

Results

The results obtained from the Crystal Glass factories showed that there is poor correlation between the total amount of lead in the air and the blood of workers. However, when air samples are investigated according to constituent lead particles by diameter, the air samples can be sub-divided according to particle size to identify the best correlation. Very strong correlation occurs for Dc = 0.2 µm, meaning that the ultrafine particles below 0.2 µm can be disolved in the lung fluid to find their way into the blood stream.

The results conclude that a significant occupational health issue exists at crystal glass factories relating to ultra-fine lead particles (diameter less than 0.2 micrometer).

The consequences of not monitoring ultra-fine lead particles in the air at work stations of crystal glass factories are that:

  1. The likely threat to workers will remain unquantified and any abatement processes unmonitored.
  2. Assessments of the level of lead in the air from only larger particles may overestimate the risk to workers.

The results show that a considerable proportion of lead (in mass %) is contained in ultrafine particles less than 0.2 micrometer aerodynamic diameter. One of the Crystal Glass factories sampled actually contained more than 75% of the lead mass in the region of ultrafine particles (diameter less than 0.2 micrometer).

The more usual size distribution has a maximum at 1-2 micrometer with a secondary peak of lead associated with ultra-fine particles in the range from 0.01 to 0.1 micrometer emphasising the importance of monitoring ultra-fine particles. See figure 1, below:

Figure 1. Lead mass aerosol size distribution (dm/dlogD) obtained at a crystal glass factory.

Conclusion

Ultra-fine lead particles represent a considerable proportion of lead mass in the air of a factory that involves heating of lead or its compounds. Size resolved sampling is necessary to evaluate air quality at working places to monitor the extent of the health risk to workers and maintain effective prevention or abatement methods.

 

 
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