Week 3 Flashcards
acute
- short term bursts of exposures (hours to days)
- often higher concentrations/doses
- may be followed immediately by symptoms or exacerbation of existing condition
chronic
- longer-term periods of exposure (years to lifetime)
- often lower concentrations/doses
- may be associated with onset of new disease (sometimes after long latency period)
form of exposure routes
Three routes of exposure
- dermal
- inhalation
- ingestion
- (also mother-> fetus)
Human barriers to the environment
- skin
- respiratory tract
- GI tract
Exposure route will influence both how you assess exposure and, potentially, how you intervene to reduce exposure
The environmental risk paradigm (exposure pathway - pollutants)
pollutant emissions -> movement of pollutants -> exposure -> dose -> effect
- pollutant emission, movement, exposure and dose are exposure assessment
what is exposure pathway?
The physical pathway a pollutant moves from the source to the subject
- how it moves through the environment (air, water) and how it enters the body (inhalation, ingestion, skin contact)
what is exposure route?
The way a substance enters the body
- inhalation, ingestion, or skin contact
Exposure pathways: where to intervene (3 intervention spots)
- at the source
- along the pathway
- at the household or individual
Air pollution example:
Pollution released into the environment (reduce emissions) -> movement of pollution (taller smoke stacks) -> exposure (air cleaners) -> (masks) -> dose -> effect (medical interventions)
How to reduce hazard exposure?
Produce less of that hazard
- less carbon emission (electrical cars, transit)
3 key ideas in exposure assessment
- concentrations vary in time
- temporal variation - concentrations vary from place to place
- spatial variation - people move around
- we want to know: what is the concentration where people are and when they are there?
Exposure assessment methods - direct methods
Includes:
- personal measurements
- biological markers (biomarkers)
Tradeoffs:
- can provide a better estimate of true exposure
- expensive and time-consuming
- high demand on participants
- often not feasible on a large scale
Exposure assessment methods - indirect methods
Includes:
- “area” measurements
- questionnaires
- models
Tradeoffs:
- may provide a poorer estimate of true exposure
- less expensive and time-consuming
- little to no participant demand
- can be applied to large populations
Direct methods: Biomarkers
Measure pollutants or metabolite in a biological material
- exhaled breath, urine, blood, teeth, hair
Tradeoffs:
- estimates dose and incorporates multiple routes of exposure
- can be intrusive, interpretation can be difficult
Indirect - Questionnaire-based exposure assessment made possible by
- knowledge of exposure: exposure of interest in this case was self-administered
- exposure of interest administered in consistent, quantifiable “units”
- stable exposures: smoking patterns don’t typically fluctuate widely over time; people tend to smoke in consistent patterns (amount, type)
Dr. Irving Selikoff (1915-1992)
- 1950s, several workers from a nearby asbestos factor sought treatment at Selikoff’s clinic
- he tried to get the workers medical records from their employer, but was denied
- working through the labor union, he was eventually able to obtain the workers records
epidemiology definition
The study of the distribution and determinants of disease in humans
3 key ideas in epidemiology
- not everyone who is exposed to a hazard will develop disease/illness from that exposure
- some kids who ate cold chicken did not get sick
- some people who smoke do not get lung cancer - Not every case of a disease/illness is the result of the exposure of interest
- some kids who did not eat cold chicken got sick
- some non-smokers develop lung cancer - Correlation is not the same as causation
- umbrellas don’t cause rain
- ash trays don’t cause lung cancer
Correlation does not equal causation
- the fact that A and B are correlated does not necessarily mean that A causes B or that B causes A
- causation can be difficult to prove, and observational studies alone generally cannot prove causality
-> experimental designs can provide stronger evidence of causality - the most compelling evidence often comes after replication and when the epidemiological and toxicological evidence converges
observational study designs
- cross-sectional
- case-control
- cohort
- time series
Experimental study designs
- RCT
- natural experiments
Experimental designs are uncommon in environmental and occupational health research
RCT - the ‘gold standard’ study design
- questions about external validity
- can be difficult to “blind” participants
- ethical concerns
-> usually we can only reduce exposure
-> should evaluate interventions and outcomes relevant to the population - usually evaluating exposure reductions at the individual or household level
-> generally not feasible to randomise cities, provinces, or countries to policy changes - generally not useful for evaluating diseases with long latency periods
-> acute or sub-chronic exposures, well defined exposure “window”
Cross-sectional study design
- “snapshot”
-> assess exposure and outcome at a single point in time - often a good place to start, “hypothesis-generating”
-> quick, inexpensive - not good for providing evidence of causality
-> temporal relationship between exposure and health outcomes is unknown
Cohort studies
- identify participants who do not have the disease of interest, observe them over time to assess new cases of the disease
Advantages:
- generally less susceptible to bias than cross-sectional or case-control
- temporal relationship between exposure and outcome is known
- can study rare exposures
Disadvantages:
- expensive and time-consuming (prospective cohort)
- not useful for studying rare diseases
case-control studies
- identify participants based on disease (Outcome), then determine who previously had the exposure
Advantages:
- often, relatively small sample sizes are adequate
- can study rare diseases
Disadvantages:
- exposure assessment is difficult (retrospective)
- not useful for studying rare exposures
odds ratio
the ratio of odds in favour of exposure among the cases to the odds in favour of exposure among the non-cases
4 Key ideas for study designs
- one study is usually insufficient
- studies of studies - effect estimates (relative risk, odds ratios) can be wrong
- bias (selection, information, confounding) - effect estimates (relative risk, odds ratios) only tell part of the story
- individuals vs populations
- the high risk or clinical strategy vs population strategy - when do we have enough evidence to act?
Effect estimates (relative risk, and odds ratios) can be wrong
3 broad categories:
1. confounding
2. information
3. selection
- key idea: effect estimates in poorly conducted epidemiological studies can be wrong
- so we must consider not only the result, but details of the study that produced the result
individuals vs populations
two key ideas:
1. prevention paradox
2. shifting the curve
These ideas are very relevant because of the ubiquity of exposures
- these ideas are important in environmental health because so many people are exposed to the environmental hazards
- for some hazards and populations, 100% of the population may be exposed
population attributable fraction
Population Attributable Fraction (PAF) is the percentage of disease or deaths in a population that could be prevented if a specific risk factor were eliminated or reduced to an ideal level.
prevention paradox
the in the high-risk (or clinical) preventive strategy we go out and identify those at the top end of the distribution and give them some preventive.. but this high-risk strategy, however successful it may be for individuals, cannot influence that large proportion of deaths occurring among the many people with a small risk
- we are therefore driven to consider mass (population) approaches, of which is the simplest is the endeavour to lower the whole distribution of the risk variable by some measure in which all participate
Translated into environmental health context: reduce exposure of entire populations to environmental hazards
individuals vs populations - shifting the curve
we only shift the curve if exposure prevalence is high… as is often the case with environmental hazards
how should we work to prevent death and disease from environmental hazards?
high risk strategy vs population strategy
when should we act to prevent death and disease from environmental hazards?
precautionary principle vs firm thresholds for causality
when do we have sufficient evidence to act? precautionary principle
where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation
OR
when an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically
when do we have sufficient evidence to act? - definitive causality
- temporal relationship
- strength of association
- dose-response relationship
- replication of findings
- biologic plausibility
- cessation of exposure
- analogy
- biological coherence
What does shifting the curve mean?
- Shifting the curve refers to reducing population-wide exposure to a harmful factor by lowering the overall distribution of risk, rather than just targeting high-risk individuals.
- This approach focuses on small improvements across the entire population, leading to significant public health benefits.
