Resources Section 2: Methods of estimating health risks due to environmental hazards

Section 2:  Methods of estimating health risks due to environmental hazards

Aim of this section

This section of the course provides an introduction to the methods used to identify environmental hazards and the health risks associated to exposure to these hazards.

Even if this section contains references to toxicological studies and statistical modelling, you do not need to focus on the mathematical and toxicological details for this OOC. You are only expected to understand the concepts broadly.

By the end of this section, you should be able to:

• Understand the principles and aims of risk assessment 
• List the different steps used to assess health risks due to environmental hazards
• Describe the key concepts of each step
• Apply the DPSEEA framework to an environmental hazard

Introduction

In section 3, you will be introduced to some environmental hazards and their consequences on health. But how do we determine which potential hazards deserve attention? How do we decide if we should spend money and resources on a particular hazard’s control? Health risk assessment is used to answer these questions. It is the process of estimating risks to health attributable to environmental hazards. It comprises of 4 main steps that we’re going to detail in the following sub-sections:

• Hazard identification
• Dose-response assessment
• Exposure assessment
• Risk characterisation

In this section, we will use a document created by the Australian Department of Health (You do not need to read it in whole. PDF files of the relevant chapters will be provided in each part of this section. Many chapters of the document are beyond the scope of this course). 

As an introduction to risk assessment, please read chapters 1.1 and 1.2 of the Australian Department of Health document.

2.1 Hazard identification
Hazard identification is the assessment of whether a substance or a product can cause harm to health. Please read the “planning” and “hazard identification” tabs on the United Stated Environmental Protection Agency (USEPA) website, as well as chapters 2.1 to 2.4 and 3.2 in the Australian Department of Health document. 

Hazard identification involves examining the available scientific data (environmental toxicology and environmental epidemiology data). Toxicological studies generally involve in vivo testing on animals to identify hazards, including:

• Reproductive tests: to identify the effect of the hazard on reproductive performances
• Genotoxicity tests: to identify if the hazard can cause DNA damage or mutations
• Toxicity tests: to identify the effects of a single exposure, of repeated exposures and of long-term exposure

Epidemiological studies are used to study the distribution and causes of diseases in humans. Epidemiological studies include:

• Case-control studies: to compare the exposure status between people who don’t have a disease (controls) and those who have a disease (cases)
• Cross-sectional studies: to measure the number of cases of a disease (prevalence) in a population and the frequency of exposure at one point in time
•  Cohort studies: to follow exposed and unexposed people over time and assess if there is a difference in disease development between the 2 groups

The results of these studies will be used to quantify the burden of disease due to an exposure, i.e. its impact on health. The burden of disease can be expressed in number of deaths, incidence of a disease (number of new cases over a period of time), DALYs… DALYs (Disability Adjusted Life Years) are a useful summary measure of population health. It is more comprehensive than the number of deaths as it combines information about mortality and non-fatal health conditions (disability). Please read information on DALYs on the WHO website.

 

As explained in chapter 2.2 of the USEPA document, environmental impacts on health are uneven across life course and gender. The health of children under five, and to a lesser extent up to 10, and that of adults between 50 and 75 years is most affected by the environment. Women bear higher exposures to traditional environmental risks to health, such as exposure to smoke while cooking with solid fuels or carrying water from community sources. However, men are more exposed to occupational risks to health.

 

Optional: For more detailed information about the different types of toxicological studies, you can read chapter 9.1 in the Australian Department of Health document. For more detailed information about the different types of epidemiological studies, you can read chapters 10.1 and 10.2.

 

2.2 Dose-response assessment

Dose-response assessment consists in determining the dose of the hazard that can cause adverse health effects and how a change in dose will influence the likelihood or frequency of these adverse effects. Please read the “Dose-response” tab on the USEPA website and for information on dose-response, watch this short video

Some values are estimated during dose-response assessment. Please read the definitions of LD50, NOAEL and LOAEL, and look at the dose-response curve shape on this website. Most dose-response curves in toxicology have a sigmoid shape. For some hazards, there is a specific dose (also called threshold) above which the toxic effect is produced and below which no adverse effects occur. The maximum dose without adverse health effect is referred to as the “no observed adverse effect level” (NOAEL) and the lowest dose at which there is an observed toxic effect or adverse effect is the “lowest observed adverse effect level” (LOAEL). In order to obtain the tolerable daily intake (TDI), i.e. the daily dose of exposure in which the chemical has been assessed “safe” for humans, NOAEL and LOAEL are divided by an uncertainty factor which depends on whether human or animal data are available.

Optional: If you want further knowledge on how the TDI is estimated, you can read chapter 5.6 in the Australian Department of Health document. 

2.3 Exposure assessment

Exposure assessment is the process of measuring and estimating the intensity, duration and frequency of exposure. Please read the “exposure assessment” tab on the USEPA website, as well as chapter 4.5 in the Australian Department of Health document.

In exposure assessment, we will usually try to answer the following questions: 

• How? (Exposure pathway= the means by which the contaminant comes into contact with the target organism such as drinking water, contaminated crops…; exposure route = how the contaminant enters the body such as ingestion, inhalation, dermal contact, placenta…)
• To what? (= which hazard, single / multiple / continuous exposures…) 
• When? (= in the past, during childhood, now…) 
• Where? (= at home, work, school…)

There are different approaches for quantifying human exposure. One method is the use of biomarkers. This is based on using some biochemical properties of the hazards. Levels of these biomarkers can be measured directly in the blood or other tissue samples. Information on the use of biomarkers can be found on the USEPA website. Another method is to assess the contaminant concentration in the medium to which people are exposed (e.g. water, air…) and to use this information to estimate the dose received by individuals. We can then calculate average daily doses (ADD) or lifetime average daily doses (LADD) received by individuals. LADD is usually preferred to ADD in case of exposures for which the adverse health effect generally occurs years after exposure (e.g. cancers). 

 

Optional: If you want further knowledge on biomarkers, you can read chapters 14.1 to 14.3 in the Australian Department of Health document. If you want further knowledge on how ADD and LADD are calculated, you can read section 2.1.4.1 (pages 10 to 13) in the USEPA guidelines on exposure assessment and this paper as a case-study showing how we can determine the LADD.

For those of you who have read the calculation methods for ADD and LADD in the USEPA guidelines document, here’s a summary of the key formulae:

2.4 Risk characterisation

Risk characterisation is the process of integrating the information from hazard identification, exposure assessment and dose-response assessment in order to quantify the risk to individuals, populations and/or the environment, and to provide an evaluation of the overall quality of the assessment. We therefore try describe the risk in terms of the nature, extent and severity of potential harm. Please read the “Risk characterization” tab on the USEPA website, as well as chapters 5.1 and 5.2 in the Australian Department of Health document.

Uncertainty and variability are part of risk assessment and must be acknowledged in order to communicate the results clearly to policy-makers and stakeholders. Uncertainty represents a lack of knowledge about factors affecting risk characterisation, whereas variability represents the heterogeneity across people, places or time.

Optional: Risk characterisation assumes a causal association between the exposure and the health effect. Because a statistic association between an exposure and an outcome does not necessarily imply causation, Bradford Hill’s criteria of causation are used to determine whether an association can be considered as causal or not. You can find information on Bradford Hill’s criteria in this WHO Bulletin.

For further knowledge about uncertainty in risk assessment, you can read chapter 5.15 in the Australian Department of Health document.

2.5 DPSEEA framework

The DPSEEA framework is an efficient tool developed by the WHO in 1996 to summarise the information obtained from risk assessment. Because it includes more immediate hazards (exposures) and more upstream causes of environmental hazards (environmental pressures and driving forces), it provides an overview of the linkage between environment and health and supports decision making.

Please read this page for general information on the framework. Here are some key information on the 6 parts of the DPSEEA framework:

• Driving forces: factors at the “macro” scale of society. It includes macro-economic policies, social and demographic developments… Some examples are urbanisation, population growth, employment rates, transport policies, energy policies…
• Pressure: the driving forces can result in pressures on the environment. Examples of pressures are emissions of pollutants from traffic / from industry / from agriculture / from household fuel, production of products containing toxic chemicals (paint, petrol…)
• State: the state of the environment can be modified by the pressures, resulting in a new state such as desertification, climate change, household air pollution, contaminated food /soil / water, indoor air pollution
• Exposure: nature of the hazard, such as carbon monoxide, carbon dioxide, particulate matter, chemicals (lead, arsenic…), frequency of exposure, quantity of exposure…
• Effect: adverse health effects due to exposure to the hazard, such as mortality, particular diseases…
• Action: actions taken by policy-makers and other decisions-makers to tackle the issue, such as switching to clean energy, banning or regulations on toxic products, improving access to fresh water. Actions can be taken to tackle the issue at any stage of the framework
  

Please also read this paper as a case-study on how to apply the DPSEEA framework on healthcare waste management.

 

Optional: This WHO document provides more detailed definitions and information on how to fill the different parts of the DPSEEA framework, as well as examples to fill each one of the 6 parts of the framework

Conclusion

To synthetize this section, please watch these 2 shorts videos:

and

You now know the key concepts of risk assessment for environmental hazards. But what is the following step? When an environmental risk has been characterised, how do we manage it? This is usually referred to as risk management and usually involves policy making. Environmental policies will be introduced briefly further in this OOC (Section 4).

Moreover, when there is reasonable suspicion of harm, but lack of scientific evidence or consensus, uncertainty must not be used to postpone preventive action. It is called the precautionary principle. It enables a rapid response in the face of a possible danger to human, animal or plant health, or to protect the environment.