Assessing Health Effects

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Health Effects Assessment


Environmental Health Effects measurement

Prologue

As the Indian Airlines flight landed in the Birsa Munda airport in Ranchi, India, we tightened our seat belts and looked out of the tiny aircraft window. The Roro hills rose in the horizon as the aircraft turned towards the tarmac. Our journey began.
We climbed into the car that Peter sent for us and we were greeted with the lush greenery around us and the hill that was out there, where company X mined Asbestos for years and then abandoned the hills and the little township. The following morning, the five of us were joined by a journalist, Mimi, and we went up the hill along a little dirt track. The climb was steep, but it was not hard, The morning sun shown overhead, and we were sweating by midday when we went up. Around the mine, a few people greeted us. A few short, slender men were hanging about, a few women were out there searching among the rubbles of what looked like piles of blue asbestos rocks piled there. Children were playing. A lot of dust were generated and people did not seem to care.
“Study the people Arin, “, Peter asked me pointing to the men, women and children, “What would you say? These were all people who used to work the mine. The company closed the mine and moved on. Except they did not bother to close the tailing ponds properly.” Peter guided the group of us to show a few of these tailing ponds. It was obvious that the company did not bother to close them properly and the people here were exposed to high concentrations of asbestos both in the air and they were playing with the piles. Something needed to be done, urgently.
In the afternoon, we came down from the hills, and organised a camp where we were examining the local people from the area. They mostly complained for breathing difficulties, some had severe backaches, and what you’d consider pretty regular stuff, routine complaints. As we we examined the people and listened to their stories, it turned out that nearly all of them suffered from some from of form of respiratory problems, and people who worked in the mine for years also had severe backache and severe bodily pains. They were also hungry and tired.
In the afternoon, we met with the local doctor and the chief medical officer in Roro hills areas. It was astonishing to see the hand waving and wishing away that this guys were showing. Between our cups of chai, the doctor assured us that these are were all poor people and what we were seeing were just good old tuberculosis. Nothing to do with asbestos mining.
Then out popped an Xray plate with almost like a small coin shaped opaque shadow across the chest and streaks of white strands in the fields. Mimi was insistent, what these could be. The doctor smiled, and said something like “could be asbestosis”.
Could be? There was silence. Someone discovered blood stains in the beds of patients they saw in their rounds in the hospital that morning. A story of mass exposure to asbestos was unfolding.

Conceptual Introduction of Measuring Health Effects

All environmental exposure lead to one or other health effects; some of these are undetected or subtle, others have manifestations ranging from minor discomforts to death. Health effects in individuals and populations can be measured in a number of different ways. At the individual level, this could be symptoms and signs. Symptoms refer to “complaints” that individuals verbally express to their health caregivers (nurses, doctors, other caregivers). The term “sign” refers to the findings that a doctor discovers upon examining a patient. The signs and symptoms taken together lead a clinician to order more tests and additional investigations for the individual to confirm a diagnosis before treatment can be prescribed. At the level of populations, these different signs and symptoms are considered together in the form of diagnoses and the diagnoses are tallied. Occasionally, the disease conditions or biological or physiological parameters are summarised to indicate the level of health states of entire populations. When exposure to an agent in the environment results in physical harm, the process is known as toxicity. An exposure can therefore a toxic exposure if it is capable of resulting in physical illnesses that can result in significant disability or even death of a number of individuals.

Toxicity

Toxicity refers to the capacity of an environmental variable to cause harm (EnvironmentalEpidemiologyA 1999). Toxic agents target either a tissue or a specific set of target molecules; toxicity in turn can develop over a short period of time (acute or toxicity), or it can take time to develop (chronic toxicity). For example, recently Jonathan Klick and Joshua Wright (2012) wrote a concept paper where they claimed that reusable cotton bags that are often used in lieu of plastic bags used for shopping harbour E coli bacteria, and these grow if the bags are kept in the trunks of cars (Klick 2012) . In turn, they argued that rather than using cloth bags for shopping, it’d be more cost effective if shoppers continued to use plastic bags. But not here the argument: the cloth bags are used to carry food items; as a result, E coli contaminate the food and in turn when these foods are consumed either cooked or raw, E coli from the food enter the body where they result in clinical manifestations of diarrhoea, vomiting, and fever. These events occur relatively rapidly, over days, and hence this toxicity is termed as acute toxicity. On the other hand, people may be exposed to high concentrations of air pollutants for years, and over years, some people manifest signs of respiratory difficulties, but these occur following years of exposure. While acute or chronic toxicity refers to the state of development of adverse health effects over time and the speed of this occurrence, it is also important to appreciate that effects can span quite a range of manifestations (the spectrum of effects) which we study now.

Spectrum of Effects

At one end of the range of effects (spectrum) is “no effect”, that is, the person is either healthy, or complains of minor discomfort; at the other extreme, an individual can die. For example, on days of high pollution, some people complain of irritation in their eyes and nostrils. While these present significant discomfort for these individuals, none of these health effects are life threatening. Some changes are subclinical changes where subtle mechanisms occur and these may be the first or initial signs of changes that occur but clinical manifestations may not occur at all; however, understanding subclinical changes are also important as these enable actions to be taken earlier than clinical manifestations and can lead to better prevention.

On the other hand, with acute toxicity, think of what happened in Bhopal in the night of Third December, 1984. On that day, immediately 3, 800 people who lived near the Union Carbide Plant in plant in Bhopal, India, died as they were suddenly exposed to very high concentrations of methyl isocyanate gas that leaked out of the plant, and then entered their body bodies (Broughton 2005).

Something similar happens when people are exposed to high concentration of carbon monoxide gas from burners and incomplete combustion and retrograde flow of these gases (Prockop 2007). Between these two extremes of discomfort and death, other health effects or health conditions make up the spectrum; in other circumstances, people may suffer from loss of functionalities. For example, Sobngwi and colleagues (2004) suggest from their research in Africa that urban enviromment and migration to urban environment and environmental stressors can be implicated in the emergence of hypertension (high blood pressure) and diabetes (Sobngwi 2004). The point here is this that, exposed to environmental stressors, some people might develop diabetes, yet others may not develop any disease at all. This suggest that a range of reactions as well for individuals who are exposed to the same levels of environmental stressors. Why could this be? One possible explanation is that, as people are genetically diverse, these could be related to how their genes and environmental variables interact to produce the health effect or react to the environmental stimuli.

Relevance of Genetics to Environmental health

Genes are complex to define. At a conceptual level, think of genes as units of heritability that physically consist of “strings” of triplets (each triplet is a combination of three out of four nucleotides: Adenine, Guanine, Cytosine, and Thymine), and functionally, genes code for proteins that perform different functions in the body (resistance, carriage of molecules, conduct biochemical reactions). Each of us have about 300, 000 genes distributed in 23 pairs of chromosomes (22 chromosomes are known as autosomes and one pair of sex chromosomes).

Our environmental conditions impact by changing genetic structures and functions in different ways. Some of these changes go unnoticed as no major changes occur. Others manifest in the form of genetic disorders. Some genetic disorders occur are due to arrangement of the chromosomes themselves. For example, Down’s syndrome is a disorder characterised by specific physical feature and mental health issues. Here the problem is trisomy of the chromosome 21; instead of a normal of two copies of Chromosome 21, we get to see three copies of Chromosome 21. Genes also undergo changes in the form of mutations or single nucleotide polymorphisms. In single nucleotide polymorphisms, one out of four possible nucleotides Adenine, Guanine, Thymine, and Cytosine at a particular locus on the chromosome/gene is substituted or replaced by another. This can also be say for example an additional insertion of a nucleotide or a loss of a nucleotide. These in turn lead to a different configuration of the gene and thus, gene product. SNP is labelled where the prevalence of such change is more than one percent in the population; if the prevalence of these changes are less one percent in the population, the phenomenon is known as mutation (Wild 2005).
Numerous environmental mutagens have been identified. For example, Ahsan et al (2007) reported that exposure to inorganic arsenic in drinking water often show single nucleotide polymorphisms (Ahsan 2007). People who live in radiation hazard sites are at high risk of mutation and therefore manifest different types of genetic disorders. For example, in the state of Jharkhand in India, in a place known as Jadugoda, uranium were mined for years. Exposure to radioactive minerals from the uranium mines have resulted in significant health effects for children and people who live in the area.
In addition to direct impact on genetic structure and therefore alteration of gene functions, epigenetic changes also occur as a result of exposure to environmental agents. In epigenetic changes, heritable changes occur even without any alterations in DNA sequence (Baccarelli 2009). In general, three common mechanisms are proposed: these include changes in histone structure of the chromosomes, or methylation of DNA, or formation of micro RNAs.

Acute versus Chronic Effects (latency)

We discussed in the above sections that health effects occur as a result of being exposed to specific environmental agents. We discussed that such effects can be toxic and these toxic effects have a range of effects and this range in turn can be explained on the basis of the environmental effects on the genetic structure and functions. There are also issues around health effects occurring over time. In this conceptualisation, health effects can be acute versus chronic. Acute effects are those that occur over a short period of time. For example, consider the case of people who are exposed to sudden high concentrations of carbon monoxide from incomplete combustion (trapped on a snowy night in a car and could not get out); unfortunately they die within a short span of time being exposed. As another comparable example, people who were exposed to heavy concentrations of methyl isocyanate from the Bhopal plant, died almost instantly after being exposed to such high concentrations of the gas. Such health effects are known as acute effects.

On the other hand, some health effects take a long time to develop. For example, those who are exposed to inorganic arsenic through their drinking water supply, the initial manifestation of health effect in the form of skin lesions often take years, typically anywhere between five and ten years (Smith 2000) . As another example, Bianchi et al (1997) estimated latency period of 300 individuals in various trades exposed to Asbestos and found that the latency period of appearance of mesothelioma can range between 14 to 72 years (Bianchi 1997).

For most environmental toxins, the onset of disease and its manifestation takes time. This interval can be partitioned into two phases. The initial phase is referred to as the induction phase where from the time of exposure a time gap is observed till the biological process begins which can be manifested in altered metabolism, for instance. The disease has not yet manifested clinically; this stage is referred to as subclinical phase of the disease; then, from the first point of subclinical manifestation till the onset of the symptoms and signs, the time interval is referred to as latent period (Figure 1). For some disease and for some exposure both the subclinical and the latent period are short; for example, for influenza, both the latent period and the subclinical phases are short. On the other hand, cancer causing agents typically have a long latent period.

Specific versus non-specific effects

For some health effects, it is not difficult to identify the possible environmental exposure that preceded it. For example, people exposed to high concentrations of arsenic (either in the drinking water for long periods or from other sources such as occupational exposure to copper smelting for example), manifestations of skin pigmentation occur after a long period of time but the manifestations themselves are quite specific – these include very specific dark and brown pigmentation on the skin of the back. Other health effects are generic. For example, people who are exposed to asbestos develop symptoms and signs of respiratory illnesses that resemble tuberculosis and are often misdiagnosed with tuberculosis or silicotuberculosis.

Susceptibility

The term susceptibility refers to the proneness of an individual to manifest a particular disease or health effect, more or less when compared to other individuals who are also exposed to the environmental variable. For example, it is known that the malarial parasite known as Plasmodium falciparum causes a particularly virulent form of malaria. Yet not all individuals exposed to the same parasite under the same environmental condition will develop falciparum malaria. Individuals with specific blood groups (Bombay blood group) and serotypes that even when exposed to P falciparum will not develop the disease. Another example is gluten enteropathy. Conleth Feghiary argues that much of coeliac disease is still unclear. People who are susceptible to glutens will develop a very specific type of intolerance to glutens and show symptoms and signs of enteropathy(Feighery 1999). Hyperreactivity and hypersensitivity are two forms of susceptibility patterns which we discuss next.

Hyperreactivity versus Hypersensitivity

Hyper-reactivity and hyper-sensitigity refer to the situation when people are exposed to specific agents, they develop severe health effects and responses characterised by increased flow of blood and often features of shock or loss of blood pressure and sudden collapse. For example, when exposed to specific allergens in food or in the environment, people develop rashes, skin inflammations, and inflammatory conditions in the bronchial tube (asthma). The first time individuals encounter an allergen, the reactions may be minor, but once sensitised, when they are exposed a second time, individuals may show very different and severe form of response, and quite often these reactions are life threatening. Anaphylactic shock is a condition that occurs due to extreme hypersensitivity to specific environmental agents such as drugs or pollens or specific animal products.

Thus far, we discussed some issues around environmental exposure and resulting diseases – how they developed, the time span over which they develop (acute versus chronic), and the variability of their manifestations (severe versus less severe) and we looked at the susceptibility of individuals and populations. Let’s start reviewing how we can measure health effects.

Case Definition

In health effects assessment, a precise case definition is the first principle. The term “case definition” refers to the process of clearly setting criteria for selection or exclusion of of a disease process. A precise case definition should allow one to identify a case of disease when these signs and symptoms are observed, or rule out diseases and health conditions or health states. During outbreak investigations, this is the first step that epidemiologists start with.

How Might We Measure Health Effects?

Several issues need to be considered when we embark on answering this rather broad based question. We review data sources, and linkages for both primary and secondary data. We review sources of both secondary data and primary data sources – databases, and collecting primary data from individuals and samples drawn from larger populations.

Data sources (primary versus secondary data)

A major source of population based health effects data is that data collected and stored in the form of secondary data such based on large scale health surveys conducted by the Ministry of Health or the World Health organisations or other organisations or reported in the literature. As these data are not collected primarily by the researchers themselves but the researchers rely on other sources to obtain and source the data, therefore these are referred to as secondary data. Primary data, on the other hand, refer to the data obtained from first hand (“primary”) surveys such as cross sectional surveys or studies conducted such as case control studies or cross sectional surveys on specific health related states. In the following paragraphs,we discuss several different sources of secondary data

Data linkages

The term “data linkage” refers to the condition where individual data sets are connected with each other using a range of different identifiers and connectors. For instance, let’s say we have obtained a data set on climate variables and we have also access to another data set on hospital admissions from the same region or city. For example, in large scale air pollution studies that link ambient air quality and different health parameters, data on air pollution or ambient air quality are collected from individual stations for a particular city over a period of time. Then for the same city, researchers collect data on hospital admissions (could be due to asthma or cardiovascular illnesses) from different hospitals. Then the two datasets are then joined or linked on the basis of finding a common identifier.

Sources of Mortality and Morbidity Data

Mortality Data

The death or mortality data are commonly sourced from the death register registers. These might include: hospital or morgue records. In addition, for different countries, the State collects such data periodically and makes them available to investigators and researchers and health professionals. The common measures are death rates, or age-specific death rates.

Morbidity Data

Human illnesses or health related states data (morbidity data) are obtained from the ministry of health databases, or from individual hospitals or workplaces, and schools or indeed other areas where some officials keep record of people’s health status as they enter to interact with businesses or do their activities. For hospitals, this would mean patients who attend the inpatients or outpatients departments. In small clinics, this might mean all those who attend the various clinical facilities. In the context of the workplaces, even before the employees enter the job, pre-employment checks are made. Then, when the employees start their jobs, another round of examination of health states are made and the data are maintained. During the various phases of employment health checks are conducted. Thus workplace records may provide a rich source of health records from where health data can be abstracted and studied. Similarly with school based physical records and health checks, records can be accessed and studied. In addition to these commonly available sources, disease registries such as congenital disease registries and cancer registries are common sources of data for health care data analysis. The common measures used for morbidity are prevalence, incidence, age-adjusted and age-standardised rates of incidence and prevalence.

Hospital Records

Hospital records are maintained when a patient first reports to the hospital or are referred from some source to the hospital. During these clinical encounters, hospital clerks or the clinical personnel enter data on their age, gender, socioeconomic status, eligibility or availability of insurance, and from the clinical side, detailed history and records o of physical examination, and instrument based acquisition of data (that is X-rays, or ultra sonograms, or other means) are taken and the records are then maintained.

Employment Data

Before joining work, at many workplaces, employees are routinely examined for their physical fitness or checked for the presence of specific illnesses that are relevant to the job they are joining. For example, the physical fitnesses are routinely tested for people in the police force. These data frequently include respiratory flow, vital capacity and other tests. Many places also test the employees for their blood sugar level, high blood pressure level (thence hence they measure their blood pressures periodically), and vision are tested as well. Many people who work for software companies (where the job stress may lead to complaints such a carpal tunnel syndrome or other orthopaedic disorders, it is common for the employees to undergo orthopaedic check ups). The these data can be accessed or a be made luna aa available to the employees or researchers upon request. These data may also be made available on in the form of anonymised records.

Disease registries

Disease registries are common sources of health or morbidity data. Many countries keep registries for diseases that are either rare or diseases that o are of special interest. For example, many countries maintain birth disease or congenital disease registries. Most countries in the world maintain cancer related disease registries. Internationally, the IARC (International Agency for Research on Cancer) section of the World Health Organisation coordinate and share data among different cancer registries. In the cancer registry, the and other health providers are provided forms where they will have to enter details about the cancer diagnosis of a patient. Cancer disease registries also conduct health surveys or cross sectional surveys from time to time.

Health Interview Surveys

Many countries and the international agencies periodically conduct cross sectional surveys. These are cross sectional surveys are based on health interviews. In these surveys, a representative sample of the individuals from the population are surveyed for their specific health states. Data from the health surveys can be combined with different ways with various other data sources.

How to draft questionnaires for health effects data collection

Questionnaires are important means to access and collect health related data. Some principles of constructing questionnaires are as follows:

  1. It is important to avoid leading questions, in the form of direct questions (“Are you a smoker?” as opposed to “How many cigarettes/day you smoke” and then providing a series of choices)
  2. Demographic and income level data are collected at the end of the survey as people do not like to disclose them at the outset and seeking these information may put off some individuals.
  3. The questions need to be simple and clear, and easily understandable
  4. Frame the questions using multiple choice answers and within the scope of multiple choice questions, the choices are made in a way that enable tapping of the opinion of the person answering the survey.

All survey questionnaires need to be validated before using in the field. There are three forms of validation: face, content, and construct validation. The first and the easiest form of validation is the face validation. In face validation, the questionnaire is presented to lay people and their opinions on the the readability and presentation of information are sought. If there is evidence of agreement among the different users of the questionnaire that the questionnaire is easy to fill in and intuitive and can express what is needed clearly, then the questionnaire would have passed the face validity test. Content validity is established by providing the questionnaire to experts who will rate that questionnaire on the basis of whether the items in the questionnaire can tap all the important concepts that the questionnaire is set to measure or covers the most important content areas or whether the questionnaire is missing in any specific content that must be included given the purpose of the questionnaire or the data collection exercise. The extent to which the experts can agree on the this particular aspect of the questionnaire is determined using an agreement statistic. Finally, using a preliminary survey, the constructs or the concepts on which a questionnaire is based are identified. This process is referred to as establishment of the construct validity of the questionnaire. When the face, content, and construct validity are established, then the questionnaire is ready to be deployed in the field as it has been formally made internally valid.

Physical Examinations

For individual data collection, physical examinations are important. In particular for clinical examination, physical examinations immediately follow history taking. Here, a doctor actually examines a patient and takes notes. An example would be a doctor measuring a patient’s blood pressure using a sphygmomanometer and stethoscope. These the results of the examination are noted in the form of physical examination report sheet or history sheets, and then these are made available or used for deciding the differential diagnosis of a patient.

Physiological Measures

Several different types of physiological measures are also taken to confirm the initial diagnoses arrived at for individual patients. For example in the planning of care for diabetics, doctors measure glycosylated haemoglobin, where non-enzymatically glucose to haemoglobin is measured to test the extent of glucose control for patients with long standing diabetes or those with diabetes who are tested for adherence to treatment or response to treatment for the life span of red blood cells (roughly 120 days). Another example might be use of forced expiratory volume measurement of patients to test for lung functions.

Laboratory Testing

Laboratory measures are commonly done to establish diagnoses and assess prognoses of patients. Samples are obtained from almost every system of the body an and can be tested in the laboratory to arrive at estimates of whether a person is suffering from one or other diseases. For example, thyroid function tests (measurement of tri-iodo-thyronine, T4, and thyroid stimulating hormone levels) are commonly done to test the functional status of the thyroid glands. HbA1c test is another example.

Biomarkers and Genetic Markers

The laboratory testing services can provide information about different biomarkers of exposure and disease conditions and can provide in important information about the levels of control following establishment of treatment or policy or implementations. The biomarker based assessments are based on the assumption that bodily functions can be assessed by measuring surrogate identifiers that are present in the blood or breath or other body fluids in sufficient quantities and can provide a clue as to the functional status of the different organs and organ systems.

Genetic markers are used for identifying and testing genetic basis of the disease or health conditions. For example karyotyping is a process where chromosomes are mapped and laid out. This is done to identify chromosomal abnormalities. In other cases, segments of genes are spliced and amplified using procedures such as polymerase chain reactions (PCR) and these segments are then tested for mutations and presence of genetic variations. Such procedures are based on genetic markers of specific diseases or health related states that are being discussed.

Methodological Issues in Health Effects Measurement

Validity and Reliability

The term “Validity” refers to the concept that we should be able to measure what we set out to measure. For example, if HbA1c is used to measure the extent of blood sugar control in the intermediate term (about four months), then, this is what is it should measure. Likewise, if the test is or the survey instrument is aimed at measuring a o person’s opinion or mental state then that the instrument should accurately measure the mental state and nothing else based on the constructs that are chosen for that measure.

Reliability of an instrument or a procedure refers to the fact that if the process is repeated with the same instrument or the same procedure with the same set of individuals within a reasonable interval of time when the conditions are unchanged, then in sequential results, the for the same condition and with the same set of people, the results will be very similar if not identical. For a specific disease condition or measuring or mapping health states, the key assumption is that, the measure must be both valid and reliable. Let’s see how this relates to two sources of variation we can expect.

Inter-individual variability

Inter-individual variability refers to the variation that is observed when two or ore more individuals are given the same test and the extent to which their responses differ in these tests.

Intra-individual variability

Intra-individual variability refers to the situation where the for the same individual, if the test is repeated over a period of time, the extent to which the results will vary. The extent to which the results vary may depend on a number of issues. It can be that the instrument itself was not very good and perhaps ambiguous and so while the individual provided one set of answers at time point A, he or she was prone to understand the questionnaire differently and provided another set of responses in time point B. This resulted in the instrument returning in two different sets of answers in two different times, although the conditions did not change. On the other hand, it is possible that the test are repeated after a long time and in the meanwhile the people on whom the tests are repeated also changed. When this happens, the measurements are different for the same person over two different periods of time. This is known as secular change.

While designing a measurement tool, both these issues need to be kept in mind for the tool developer. For the same individual with the same condition, and if the conditions do not change much, over a short period the measurements cannot vary much. Similarly, for a sensitive tool, for the range of individuals with roughly same conditions, the measurements returned must be similar an dam and cannot vary too much.

How Environmental Health affects different organ systems

Different environmental factors impact human health and human organ systems in different ways.

Skin and Skin disease

Sunlight or UV rays impact skin pigmentation. UV rays are also responsible for skin cancers. Exposure to arsenic causes depigmentation. Agents that damage the liver (hepatotoxicity) results in yellow discolouration of the skin (jaundice). Structurally, the skin has several layers (epidermis, dermis, and hypodermic fat). The dermis contains the roots of the hair follicles, and other elide epithelial tissues. The dermis region also contains blood cells and connective tissue. The epidermis layer contains squamous epithelial cells and melanocytes. Each of these tissues or tissue systems can be get impacted by differ went different environmental stimuli and therefore get impacted differently. Arsenic, mercury, other heavy metals, UV radiation, fungal infections, corrosive agents – all of these present different extent to which skin is affected as a result of environmental exposure.

Respiratory system and respiratory health effects

Respiratory system starts with the nostrils or oral cavity and continues with larynx, pharynx, the trachea, the bronchi, bronchiole, ending up in alveoli. The alveoli are connected across the lung parenchyma and the surrounding vascular spaces. Toxins can cross over across the alveolar boundaries, for instance, air pollutant particles, and asbestos fibres. The bronchial muscles can be hypersensitive to allergens and react to smoke. Exposure to cigarette smoking and industrial smokes and industrial agents are responsible for different types of interstitial lung diseases.

Central and peripheral nervous systems and associated illnesses

The nervous systems include the central (brain and spinal cord), the peripheral (the sensory and motor nerves that branch out of the spinal cord), and the autonomic nervous systems (some cranial nerves – the vagus and accessory nerves, and the sympathetic nervous systems). Several environmental e agents impact neve nervous system. For instance, mercury can impact the peripheral nerves; environmental agents that can cross the blood brain barrier can impact the brain and the central nervous systems. Electromagnetic radiation from cell hon phones for instance is known to impact central nervous system.

Gastrointestinal system and associated diseases

These include biological vectors such a as E. Coli and other bacteria and viruses. Liver is affected by exposure to industrial solvents. When industrial solvents come in contact with skin or inhalation of the fumes which then reach the liver via the blood stream, affect the liver (hepatocellular cancer). Inorganic arsenic can be both inhaled and ingested and this then reaches the liver, and biotransformed into methylated arsenicals, can damage the DNA and lead to health effects. The health effects include skin lesions and cancer of the different organs. Exposure to hepatitis virus as an occupational risk for people who work with blood transfusion and nurses and surgeons who need to deal with blood blood and body fluids.

Hematological disorders

Several environmental agents cause damage to hematopoietic tissues. Haematopoietic tissues are those that are involved in the regulation of blood formation: typically these tissues are in the bone marrows or in the kidney (erythropoietin generating tissue). High levels of radiation can damage the haemotatopoeietic tissues. This lead to either severe anaemia and leukaemia. For example, it is known that benzene is a bone marrow suppressor, and thus chronic or long term exposure to benzene leads to chronic anaemia. Exposure to benzene usually occurs in laboratory technicians who work in pathology laboratories that handle benzene dyes (and also occupationally in other groups where benzene or toluene dyes are handled). Petrol pump attendants are also exposed to benzene from the fumes out of the petrol tanks during filling.

Immunological disorders

Immunological disorders refer to those disorders where the immunological system of the body is at risk or compromised in action. For example, infection with agents such as HIV can lead to inn immunological disorders or immunosuppression.

Reproductive disorders

For men, any exposure that interferes with the sperm production is important. For example xxx chemical agents are found to be associated with reduced sperm counts. For women exposure to extreme stress and hormones in food or exposure to steroid s can lead to reproductive problems in women and can lead to interference with ovulatory cycles.

Hormonal disorders

Occupational exposure to radio iodine can lead to thyroid cancer. In parts of the world where iodine content in thesis soil is low, people sure suffer from iodine deficiency goitre or hypothyroidism is common.

Environmental Cancer

Environmental cancer refers to the those cancers for which an environmental agent can be implicated. For example, it is known that exposure to high frequency microwave radiation is associated with the risk of brain tumours. Exposure to ultraviolet radiation is associated ti with the high risk of development of skin cancer among people who work outdoors.

How to investigate environmental cancer

Cancers being rare diseases, case control studies are commonly used to study the associations between environmental exposure and cancers. In case control studies, people with and without cancers are selected. Those with cancers are referred to as cases and those without cancers are referred to as controls. Their likelihood of exposure are then estimated and measured through questionnaires and direct measurements. Other study designs include secondary data analyses and ecological studies. In ecological studies, exposure levels are aggregated and cancer are aggregated and then the two aggregated measurements are regressed one on the other.

Epilogue and summary

This was a brief tour of the different types of health effects and health effects measurement. We started with the concept of the health effects measurements here, and the different spectrum along with which health effects fall. We discussed variations in health effects that op people demonstrate although they may be exposed to very similar levels of exposure. We discussed different types of study designs and health effects and ways t study health effects in populations.

In the end, the typical respiratory diseases that we found in Roro hills was due to the exposure of people to the Asbestos in the mine. We conducted a cross sectional survey to describe the high prevalence of respiratory and low back pain problems among the people who lived there and had previously worked in the asbestos mine. Asbestos is a dangerous chemical to be exposed to for prolonged period of time and can be quite hazardous as numerous studies have shown.

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