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An epidemiological study of the health risks associated with re-use of composted biosolids

Simmonds, Catherine J. (1999) An epidemiological study of the health risks associated with re-use of composted biosolids. PhD thesis, Murdoch University.

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Management of biosolids in a safe and economical way is a major issue throughout the world. One avenue for biosolids disposal is the use of biosolids in the production of landscaping products for domestic gardens. Despite the advantages of recycling biosolids as a soil amendment, microbiological contaminants in biosolids products pose a potential threat to public health.

The aim of this research was to assess the risks of exposure to microbial hazards in gardening products that contained composted biosolids. The research focussed on those microbes that cause gastrointestinal and respiratory illnesses in humans.

An epidemiological study was carried out to meet this aim. A prospective cohort study was used to determine the incidence of gastrointestinal and respiratory illnesses in people exposed to composted biosolids in gardening products. The incidence of these illnesses was compared with that in a group of people who had used similar gardening products made without biosolids.

Study participants were recruited from landscaping supplies outlets in Perth, Western Australia, when buying selected gardening products. Data for the study were collected by means of two telephone interviews. The first interview focussed on demographic information, gardening practices and use of the product of interest, and the occurrence of symptoms related to gastrointestinal and respiratory illnesses in the two weeks before exposure. The second interview collected information on the occurrence of symptoms in the two weeks after exposure and additional exposures to biosolids and other risk factors. The interviews were conducted with the main gardener in the household and with any additional gardeners if they were willing to participate. Information was also collected about relevant exposures and the health of children in the household aged five or less.

No significant association between exposure to composted biosolids and the incidence of gastrointestinal illness was observed. The adjusted relative risk (and 95% confidence interval) of gastrointestinal illness for composted biosolids users compared with users of products without composted biosolids was 0.97 (0.46, 2.06) for children aged five or less, 1.06 (0.66, 1.69) for adults and 1.08 (0.72, 1.64) for households. Because the confidence intervals around the estimated relative risk included one in all cases there was no evidence of an association between biosolids exposure and gastrointestinal illness.

A significant association was observed between exposure to biosolids and the occurrence of respiratory illness in adults. The relative risk (95% confidence interval) in this case was 4.13 (1.38, 12.42). This association was not seen when the data were analysed for children or by households. This may indicate transmission of an agent that caused respiratory illness only to those who handled composted biosolids products. The lack of association between biosolids and respiratory illness in children may also have arisen because of the small number of children in the sample population.

A limited comparison of the results of this study with those of a quantitative risk assessment showed reasonable agreement between the two methods of assessing risk. This suggests that the quantitative risk assessment model examined may accurately predict risks posed by exposure to composted biosolids. Although still limited by a considerable lack of data, quantitative risk assessment will probably become an important technique in management of risks from biosolids. Further data, particularly on pathogen concentrations in biosolids, will be needed to more accurately compare the results of the two approaches and thus validate quantitative risk assessment models. Further epidemiological evidence will also be needed.

Further research is needed to improve the understanding of the role played by biosolids in transmission of agents that cause respiratory illness. This research could take the form of further epidemiological studies, analysis of types and concentrations of viruses in composted biosolids products or development of quantitative risk assessment models. Research in all of these areas is likely to be needed. To further assess the implications of the results of this study acceptable risks of contracting illnesses as a result of exposure to biosolids need to be defined.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science
Supervisor(s): Gibbs, Robyn, Plant, Aileen and Ho, Goen
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