Waste management and public health: the state of the evidence

Introduction

Background

Waste disposal and health was identified as a priority by South West Public Health Observatory (SWPHO), in 2000. This was because changes to current waste management strategies, e.g. pyrolysis and more incinerators, may have an impact on public health. In line with their function to identify gaps in health information and to give early warning of future health issues, SWPHO wanted to find out what these impacts might be and to have an input into policy making by waste planning authorities in the South West.

Aims

These were clarified in a series of meetings with members of the South West Public Health Observatory and the University of the West of England project team. It was agreed that the following questions would be addressed:

 

1. How is waste managed in England and particularly in the South West?

2. How might waste management practices impact on human health?

3. How strong is the evidence that current waste management practices have had an impact on human health?

4. How can the evidence be used in waste management strategies and practice to protect the health of the public?

Methods

The literature search

To answer the questions above, a literature search was undertaken. The first step was a scoping exercise to find out what literature was available. Once key decisions were made to narrow down the search, a thorough search was carried out. Details of the search strategies employed are described in Appendix 1. During the literature search, a database was kept of the references found and a classification scheme developed (details available from authors). Throughout the process, there has been input from a variety of people working in the field (see Acknowledgements).

Evaluating the reviews

When carrying out a literature review, the second phase is to check whether adequate reviews already exist (CRD 2001, WHO Working Group 2000). Guidelines for assessing reviews were applied (Welsh Office 1999, WHO Working Group 2000, CRD 2001) using the following questions:

 

i)   Agreement on question to be dealt with

What is the review's objective?

 

ii)   Identification of all relevant studies

 What sources were searched to identify primary studies? Was there an explicit search strategy, an effort to include all available studies, searching of bibliographic databases, inclusion of non-English language reports?

 

iii)    Assessment of quality of the studies

What were the inclusion criteria and how were they applied?  What criteria were used to assess the quality of primary studies and how were they applied? (study question clear, exposure assessed using valid and reliable measures, health outcome assessed using valid and reliable measures, study design appropriate, data analysis takes into consideration chance, confounding and bias, conclusions consistent with the results of the data analysis).

iv)    Interpretation of the evidence

How were the data extracted from the primary studies and how were the data synthesised? How was heterogeneity between studies investigated? How were data combined? Was it reasonable to combine studies and summary results of review? Do conclusions flow from the evidence reviewed? Use of meta-analysis?

 

Meta-analysis is an approach which involves the aggregation of results from a number of published studies in order to provide a quantitative assessment of the extent to which bias might account for observed results and of the patterns, and sources of heterogeneity. A meta-analysis can only be done if the biases and confounding factors are adequately addressed in the studies and if the studies measured the same exposures in the same way and compared risk between or among similar levels of exposure. For most epidemiological studies of the health impacts of waste management systems, exposure data are missing and there is no confounding control. With the exception of one review (Pruss 1998), none of the reviews attempted to aggregate results and none carried out meta-analyses.

 

Making judgements

As most of the reviews were unable to do more than summarise the literature and its limitations, a different model was sought to make sense of the evidence. The model used to appraise the evidence in this paper is the one used by the World Cancer Research Fund to evaluate the role of food and nutrition in the prevention of cancer (WCRF & AICR 1997). The model consists of guidelines for making judgements about the reliability and strength of the evidence and was chosen because the judgements are straightforward and easy to comprehend with relatively clear criteria for inclusion.

Assessment of the strength of the evidence depends on two factors:

 

To determine the strength of an association, the concept of relative risk is generally used. (See section 4.3 for further details). However, an association, even if strong and statistically significant, is not proof of causation. To determine causation, we took the following criteria from the approach used to evaluate the diet and cancer evidence:

Making judgements

From these categories, an algorithm was developed to help to make the judgements about the evidence on the associations with health outcomes from the different waste practices.

Based on the review articles, abstracts and the primary papers found in the literature search, this algorithm was used to assign judgements to the evidence about the health impacts of landfill, incineration, sewage treatment, sewage sludge landspreading and composting.

Algorithm