Perspective Chapter: Application of Environmental Epidemiology for Exposure and Health Risk Assessment

Abstract

Environmental epidemiology seeks to understand how various external risk factors may cause or protect against disease, illness, injuries, abnormalities, or death. Environmental epidemiology evidences suggested that there is the links between COVID-19 pandemic and environmental exposures. Environmental epidemiology provide information that can contribute to rational decision-making and resource allocation by providing quantitative estimates of risks. The environmental health issues are increasing attention and emerging globally, thus raising the environmental epidemiology concept as preventive medicine. Exposure can be assessed by using direct and indirect method approaches. Exposure assessment is important for the identification, evaluation, and control of health risks in the workplace and in the general environment. Ingenstion, inhalation, and skin contact are the main pathways for individuals to be exposed to hazardous contaminants. Exposure to biological, physical, and chemical agents in the environment can cause a wide range of adverse health consequences. Health risk assessment is the process used to estimate the nature and probability of adverse health effects in the past, current, and in the future about certain pollutants. Health risk assessment is conducted in accordance with hazard identification, dose–response assessment, exposure assessment, and risk characterization. Carcinogenic and non-carcinogenic risk assessments are used to estimate health effects due to exposure to pollutants.

1. Introduction

Environmental epidemiology is concerned with determining how environmental exposures (physical, biological, and chemical factors) affect human health [1]. It seeks to understand how various external risk factors may cause or protect against disease, illness, injuries, abnormalities, or death. These factors may be due to natural occurring or anthropogenic. Currently, environmental health issues are increasing attention in the public, media, and government, thus raising the environmental epidemiology concept as a preventive medicine. WHO reported that 1.4 million deaths per year in Europe are due to avoidable environmental exposures [2].

The environmental evidences suggested that environmental exposures influence the severity and occurrence of COVID-19. Emerging evidences support the association of environmental exposures like air pollution, chemical exposures, climate, and the built environment and COVID-19 [3]. Environmental epidemiology can design a scientific-based mitigation strategy. It can communicate to the population about the potential advantage of control strategies by placing them in context of the hierarchy of control [4]. Environmental epidemiology requires refined and different skills to attain effectively across disciplines, implement appropriate designs and methods of analyses to identify causal relationships, and design appropriate interventions. If it is so, environmental epidemiology will continue to develop novel preventive strategies that improve the quality of life and save health care costs [5].

Environmental epidemiology is one of the main important tools used in environmental management decision-making process, development of environmental standards, and policy implementation owing to monitor and assess of environmental hazards in different settings, and quantify and estimate their health impact on the population at risk [6]. One of the founders of modern epidemiology, John Snow conducted the first environmental epidemiology study in 1854. He investigated that London residents who drink sewage-contaminated water were more likely to develop cholera than those who drink clean water [7].

The study in environmental epidemiology are most frequently observational in nature, in which the investigators can look at peoples exposures to environmental factors without intervening and then observes the patterns that emerge [8]. This is the fact that it is unfeasible to conduct an experimental studies of environmental factors in humans [9]. For example, an investigator cannot ask some of their study subjects to smoke cigarettes to see if they have poorer health outcomes then subjects who are asked not to smoke. Environmental epidemiology mostly used cohort, case–control, cross-sectional, and ecological studies [8].

Environmental epidemiologists often apply a set of criteria to decide the probability that an observed relationship between environmental exposure and health consequences is truly causal [10]. The criteria commonly used whether there is a causal relationship or not are determined using Bradford Hill criteria [11]:

• Strength of association: the larger an association between exposure and disease, the more likely it is to be causal.

• Consistency of evidence: the same results if repeat in different time, place and person.

• Specificity of association: the exposure causes only one disease.

• Temporality: exposure precedes outcome.

• Biological gradient: the presence of a dose–response relationship supports the causal association between an exposure and an effect.

• Plausibility: reasonable pathway to link outcome to exposure.

• Coherence: the cause and effect story should make sense with all knowledge available to the researcher.

• Experiment: Occasionally it is possible to appeal to experimental evidence.

• Analogy: the use of analogies or similarities between the observed association and any other associations.

These criteria are generally considered a guide to scientists, and it is not necessary that all criteria be met for a consensus to be reached [10].

2. Exposure assessment in environmental epidemiology

Exposure is the contact between a stressor (physical, chemical, and biological agents) and receptor. The stressors may come from point, line, or area sources and reach the public by way of matrices of air, water, soil, and foods. Risk is a function of exposure and hazard. For instance, even for a high hazard (extremely toxic substance), the risk of an adverse outcome is unlikely if the exposures are near zero. On the other hand, a moderately toxic substance may present a substantial risk if an individual or a population is highly exposed [12].

Environmental exposures can be proximate exposure (directly leading to health conditions) such as physical, chemical, and microbiological agents, and distal exposure (indirectly leading to health conditions) such as, socioeconomic conditions, climate change and other broadly scale environmental issues. Proximate exposure occur through inhalation, ingestion, and skin contact. Whereas, distal exposure can cause adverse health issues directly by changing proximate exposures, and indirect through changes in ecosystems and other support systems for human health [13].

Exposure can be assessed by using direct and indirect methods. A direct method measure of exposure is the best measure for assessing the effect of a specific substance on the target population (e.g., biological markers and personal monitoring) [14]. A direct approach accounted the exposures through multiple media (air, water, soil, food, etc) for through one study technique. However, a direct method approach the data collection nature is invasive and need high costs. On the other hand, indirect method measures of exposure have greater utility for source emission assessment and control, since they are capable of linking population health to specific pollution emission sources (e.g., environmental monitoring, modeling, questionnaires, and diaries). To obtain good quality and much information, it is often useful to combine two or more methods. For example, personal exposure assessments are often associated with questionnaires and diaries, and may also include biomarker measurements [15, 16] as depicted in Figure 1.

Figure 1.

Approaches to human exposure assessment methods adapted from [16].

Exposure assessments mainly focused on prevention. It is important to know who are exposed, source of exposure, route of exposure, levels, population with the highest risk, and health effects. The important aspects or main characteristics for the determination of exposure are the nature of the agent, the intensity of the exposure, the duration of the exposure, and the frequency of the exposure. Using these basic informations, it is possible to take appropriate measures to reduce the exposure. Exposure assessment is a complex process, involving many different professions, such as occupational hygienists, toxicologist, chemists, physicians, and environmental health professionals [17].

Exposure assessment is important for the identification, evaluation, and control of health risks in the workplace and in the general environment. Ideally, it describes the sources, pathways, route, and uncertainties in the assessment. John Snow in the 1850s noted that the apparent association between the source of drinking water and the risk of dying from cholera. Although Snow was never able to see the cholera bacteria, he understood that the disease was caused by exposure to a disease causing agent in drinking water [7, 17].

In the 1950s and 1960s, the environmental exposure and health effects began. A time-series study indicated that a disease outbreaks associated with environmental pollution, such as the London fog episode due to sulfur dioxide and mercury poisoning Minamata disease in the general population of Japan and Iraq, drew attention to the relationship between environmental exposures and public health. This brought a renaissance to the realization of the significance of environmental factors for the development of diseases [17].

Exposure to biological, physical, and chemical agents in the environment can cause a wide range of adverse health consequences. For instance, heavy metal exposure through drinking water sources is a growing global concern [18]. Assessment of exposure is an important component of environmental epidemiology research. The estimation of exposure in relation to health effects is frequently difficult, and it has generally received inadequate attention. However, afield of exposure assessment is becoming an emerging issue [1, 19].

Appropriate measures of exposure can improve the ability of a study to assess adverse effects from environmental factors. Such improvements may improve the study quality and can reduce bias, but increase cost. The exposure analysis and health outcomes must be considered together to arrive at a balanced prioritization of study requirements. One of the major example achievements by environmental exposure assessment studies is possibly the decrease in lead (Pb) exposure in the general population due to the reduction of Pb in petrol and the introduction of unleaded petrol [17]. Therefore, an effective application of exposure assessment methods may improve the results of environmental epidemiology investigations.

Exposure to potential harmful agents may lead to a wide range of adverse health effects, ranging from dysfunction, discomfort, illness (morbidity) to death (mortality) [17, 20]. The relationship between source activity, exposure, and its effect on health is described in the environmental health hazard chain as depicted in Figure 2.

Figure 2.

The environmental health hazard chain adapted from [17, 20].

3. Health risk assessment in environmental epidemiology

Human health risk assessment is the process used to estimate the nature and probability of adverse health effects in people, groups of people, and communities, in the past, current, and in the future about certain pollutants [21]. Health risk assessment is usually carried out with a systematic approaches in response to public health concerns about the increasing incidence of health effects associated with environmental hazards due to the development of industries, urbanization, and agricultural activities [22]. Health risk assessment can provide objective scientific information and contribute to allocating resources to control exposures to environmental hazards and decision making by providing a quantitative estimation of risks [23]. However, human health risk assessment does not identify specific individuals who are exposed to a chemical, compare chemicals measured in individuals or groups of people to health outcomes, and diagnose disease.

A human health risk assessment is conducted in accordance with four steps according to United States Environmental Protection Agency (USEPA) [24]. Application of the process to different types of hazards will require different assumptions, models, and methods and these must be clearly stated at all steps.

1. Hazard identification: identifies adverse effects (e.g., birth defects, cancer) that might occur from exposure to a chemical or harmful agent.

2. Dose–response assessment: examine the quantitative relationship between exposure or dose and the probability of occurrence of adverse effects in a population (response) (described in Figure 3) [25], usually derived from animal toxicity tests. Some people may be more sensitive to a substance than others may. For example, children, elderlies, and peoples with existing medical conditions are more susceptible to potential hazards.

3. Exposure assessment: it is the process of estimating the magnitude, frequency, and duration of exposure to an agent, along with the number and characteristics of the population exposed. The main pathways for individuals exposed to hazardous contaminants are: ingestion, inhalation, and skin contact [26].

Figure 3.

Representation of dose and exposure adapted from US EPA [25].

The human exposure to pollutants can calculate as follows accordingly Eqs. (1–4) which is adopted from the USEPA [24].

$$DED=\frac{\text{CxIRxEDxEFxCF}}{\text{BWxAT}}$$E1

Whereas,

DED = Daily Exposure Dose (mg/Kg-day).

C = concentration of substances, for water intake the unite C expressed in (mg/L).

IR = Intake Rate (m³/day).

EF = Exposure Frequency (days/yr).

ED = Exposure Duration (yrs).

CF = Conversion Factor [10⁻⁶].

AT = Average Time (days).

BW = Body Weight (Kg).

For oral ingestion:

$$I\left(\frac{\frac{mg}{Kg}}{\text{d}}\right)=\frac{C\left(\frac{mg}{kg}\right)xAoFxIGR\left(\frac{mg}{d}\right)xEF\left(\frac{d}{yr}\right)xED\left(yr\right)xCF}{365\left(daysinyear\right)xAT\left(yr\right)xBW\left(kg\right)}$$E2

Whereas,

I = intake of substance (food, soil, water).

AoF = An oral abserction factor or bioavailability estimate (unitless).

IGR = Ingestion Rate, for water intake the unite IGR expressed in (L/d).

For inhalation of volatiles:

$$I=\frac{C\left(\frac{mg}{\text{L}}\right)xIR\left(\frac{\text{L}}{\text{h}}\right)\text{xLRxET}\left(\frac{\text{h}}{\text{d}}\right)xEF\left(\frac{\text{d}}{yr}\right)}{365xAT\left(yr\right)xBW\left(kg\right)}$$E3

Whereas,

IR = Inhalation Rate; LR = Lung Retention factor (unitless); ET = Exposure Time.

For dermal contact with soil:

$$I=\frac{C\left(\frac{mg}{Kg}\right)xAH\left(\frac{\frac{mg}{c{m}^2}}{d}\right)xSA\left(c{m}^2\right)xAFxEF\left(\frac{d}{yr}\right)xED\left(yr\right)xCF}{365xAT\left(yr\right)xBW\left(Kg\right)}$$E4

Whereas,

AH = Soil adherance; SA = Surface area of skin exposed; AF=Skin absorption factor.

1. Risk characterization: it is the integration of information on hazards, exposure, and dose response to provide an estimate of the likelihood that any of the identified adverse effects will occur in exposed people. Risk characterization requires transparency, clarity, consistency, and reasonableness. Risk characterization brings together all information from earlier steps to describe the risks to different categories.

According to Zhao et al. [27] and Ma et al. [28], children are highly exposed to heavy metals than adults due to their physicochemical characteristics (higher comparative uptake, but lower toxin elimination rates). For example, children are usually more susceptible to a given hazardous substance and likely ingest significant quantities of soil due to their finger sucking behavior and exposure via breast-feeding and placental exposure, which are generally used as the main pathways of exposure for soil metals in children.

Carcinogenic and non-carcinogenic risk assessments are used to estimate health effects due to exposure to pollutants. The non-carcinogenic risk was estimated in terms of Hazard Quotients (HQs) for the elements and exposure routes using Eq. (5). The sum of all HQs is expressed as Hazard Index (HI) Eq. (6). If HQ or HI is found to be >1, there is a chance that a non-carcinogenic health effect may occur. In contrast, if HQ or HI is found to be <1, the exposed individual is unlikely to experience adverse health effects [24].

The Carcinogenic Risks (CR) are estimated by calculating the incremental probability of an individual developing cancer over a lifetime due to exposure to a potential carcinogen using Eqs. (7) and (8) [24]. Slop Factor (SF) converts the estimated daily intake of a toxin average over a lifetime of exposure, directly to the incremental risk of an individual and the risk of an individual developing cancer. For regulatory purposes, the level of acceptable cancer risk is considered 1x10⁻⁴ to 1x10⁻⁶ [29, 30].

These parameters were calculated using the following equations:

$$HQi=\frac{CDI}{RFD}$$E5

$$HI=\sum HQI$$E6

$$CRi=CDI\text{x}SF$$E7

$$CR=\sum CRi$$E8

Whereas, CDI=Chronic Daily Intake, RFD = Reference Dose, HQi = Hazard Quotients, HI = Sum of Hazard Quotient, CR = Carcinogenic Risk, and SF = Slope Factor.

An example is how environmental epidemiology studies in health risk assessment, a study conducted in Czech Republic on potentially toxic elements pollution in agricultural soil health risk assessment revealed that of the total sample locations 6.04% non-carcinogenic and 13.05% carcinogenic to children [31]. According to Oyola et al. [32] exposure to polluted sediments through incidental ingestion and dermal contact routes was the highest risk for receptors. Another study revealed that girls were more susceptible than boys to trace metal pollutants for both carcinogenic and non-carcinogenic risk [33].

4. Conclusions

Environmental epidemiology is one of the main important tools used in the environmental management decision-making process. Environmental health issues are increasing attention and emerging globally. Exposure assessment is important for the identification, evaluation, and control of health risks in the workplace and in the general environment. Exposure to biological, physical, and chemical agents in the environment can cause a wide range of adverse health consequences. Health risk assessment is the process used to estimate the nature and probability of adverse health effects in the past, current, and in the future about certain pollutants. It is usually carried out in response to public health concerns about the increasing incidence of health effects linked to changes in environmental conditions. An effective application of exposure assessment methods may improve the results of environmental epidemiology investigations.