Environmental Health Short Course

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 modeling, 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.

Some values are estimated during the 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:

Conclusion

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.