© 2006 British Occupational Hygiene Society Published by Oxford University Press
Letter to the Editor |
Reply
Received and in final form 4 November 2005
We thank Iavicoli and Carelli for pointing out an error in Hext et al. (2005)
. We agree that Fryzek et al. (2003) used personal full-shift or near full-shift air samples for total TiO2. This section of our paper was written before the publication of the Fryzek et al. (2003) paper and was based on the report to the study sponsors, which stated that only the long-term area samples for total TiO2 dust were used for the analysis. Unfortunately we did not spot the changes to this section and the related table in the published version.
The exposure assessment approaches used by the European and US investigators were state of the art and incorporated all relevant information. However, the approaches differed and this is reflected in the quantitative estimates of exposure reconstructed in the European study versus the high, medium and low groupings of cumulative exposure in the US study. It is also apparent in the use that the two groups made of personal sample measurements. Fryzek et al. (2003) used them to estimate relative exposure levels between job titles during various time periods (supplementing comprehensive historical plant and task information). In contrast, Boffetta et al. (2004) used the measured exposure levels to adjust estimated values obtained from a structured exposure assessment model. The modelling approach used by Boffetta et al. (2004) to reconstruct exposure for each occupational title incorporated detailed historical information about production processes (raw materials, emissions and ventilation), process changes, personal protective equipment and detailed descriptions of work activities.
The two groups of investigators were aware of each other's approach to exposure assessment and it was intended from the outset that it should be possible to combine the two data sets at least at a qualitative level of exposure. That is not inconsistent with our statement that ‘it is not possible to directly compare the average cumulative exposure to TiO2 of workers in the two multicentre studies’. We stated that it is unlikely that there is a significant difference in the average cumulative exposure to TiO2 of workers in the two studies. We were criticized by Iavicoli and Carelli for not quoting data to support this statement. This is a judgement based on the information presented in the two papers about average levels of exposure by year and job title, but we have acknowledged that the information presented in the two reports is quite different in nature i.e. estimated exposures to respirable TiO2 versus personal sample measurements of total TiO2 dust.
The authors of the letter say that it is common practice in industrial hygiene to measure occupational exposure to TiO2 by total dust sampling. That is not the case for the factories studied by Boffetta et al. (2004). Of the seven factories for which samples were available, two factories used an IOM head, three used a 7-hole head, one a 37 mm closed cassette, one a 37 mm open cassette and five used a cyclone. A number measured both inhalable and respirable dust (Cherrie, personal communication).
Iavicoli and Carelli note that the exposure assessments of the US and European studies do not specifically refer to exposure to titanyl sulphate. Solid titanyl sulphate is not made in any of the factories as a discrete product and it is believed that it is an unstable material that normally only exists in sulphuric acid solution. The normal sulphate process is to hydrolyse ‘black liquor’, which is a solution of titanyl sulphate, metal sulphates and sulphuric acid. The main solid product of the hydrolysis is a slurry of hydrated titanium oxide, which is normally filtered by Moores filters. It is believed that virtually all the titanyl sulphate is converted to TiO2 in the hydrolysis process, although there is some adsorbed sulphuric acid in the washed pulp, and it could be argued that this could be in the form of unhydrolysed titanyl sulphate. Since the products are all slurries at this stage there is virtually no dust in this area of the plant, but there are very low levels of sulphuric acid mist in this area, which may contain some dissolved titanium. Hence, we do not believe that exposure to titanyl sulphate is an important factor, but we would welcome any information from Iavicoli and Carelli that contradicts this view.
In conclusion, it is difficult to understand how Iavicoli and Carelli reached their conclusion that ‘the exposure data used to reconstruct TiO2 exposure fail to provide a real estimate of occupational exposure to TiO2’.
Syngenta Central Toxicology Laboratory, Alderley Park, Macclesfield, Cheshire SK10 4TJ, UK
ICI Group HQ North West Group Safety, Security, Health and Environment, PO Box 13, The Heath, Runcorn, Cheshire WA7 4QF, UK
Environmental Management Services, Bushey Flatt Farm, Newlandside, Stanhope, Co Durham DL13 2PP, UK
REFERENCES
Boffetta P, Soutar A, Cherrie JW et al. (2004) Mortality among workers employed in the titanium dioxide production industry in Europe. Cancer Causes Control; 15: 697–706.[Medline]
Fryzek JP, Chadda B, Marano D et al. (2003) A cohort mortality study among titanium dioxide manufacturing workers in the United States. J Occup Environ Med; 45: 400–9.[Medline]
Hext PM, Tomenson JA, Thompson P. (2005) Titanium dioxide: inhalation toxicology and epidemiology. Ann Occup Hyg; 49: 461–72.
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