Law (stochastic processes)

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Exposure assessment is a branch of environmental science that focuses on the processes that take place at the interface between the environment containing the contaminant(s) of interest and the organism(s) being considered. These are the final steps in the path to release an environmental contaminant, through transport to its effect in a biological system. It tries to measure how much of a contaminant can be absorbed by an exposed target organism, in what form, at what rate and how much of the absorbed amount is actually available to produce a biological effect. Although the same general concepts apply to other organisms, the overwhelming majority of applications of exposure assessment are concerned with human health, making it an important tool in public health.

Definition

Exposure Assessment is the process of estimating or measuring the magnitude, frequency and duration of exposure to an agent, along with the number and characteristics of the population exposed. Ideally, it describes the sources, pathways, routes, and the uncertainties in the assessment.

Exposure Analysis is the science that describes how an individual or population comes in contact with a contaminant, including quantification of the amount of contact across space and time. 'Exposure Assessment' and 'Exposure Analysis' are often used as synonyms in many practical contexts. Risk is a function of exposure and hazard. For example, even for an extremely toxic (high hazard) substance, the risk of an adverse outcome is unlikely if exposures are near zero. Conversely, a moderately toxic substance may present substantial risk if an individual or a population is highly exposed.[1][2]

Applications

Quantitative measures of exposure are used:

  • In Risk assessment, together with inputs from Toxicology, to determine risk from substances released to the environment
  • To establish protective standards
  • In Epidemiology, to distinguish between exposed and control groups
  • To protect workers from occupational hazards

Receptor-based approach

The receptor-based approach is used in exposure science. It starts by looking at different contaminants and concentration that reach people. An exposure analyst can use direct or indirect measurements to determine if a person has been in contact with a specific contaminant.Once a contaminant has been proved to reach people, exposure analysts work backwards to determine its source. After the identification of the source, it is important to find out the most efficient way to reduce adverse health effects. If the contaminant reaches a person, it is very hard to reduce the associated adverse health effects.[3] Therefore, it is very important to reduce exposure in order to diminish the risk of adverse health effects. It is highly important to use both regulatory and non-regulatory approaches in order to decrease people’s exposure to contaminants.[4] In many cases, it is better to change people’s activities in order to reduce their exposures rather than regulating a source of contaminants.[5] The receptor-based approach can be opposed to the source-based approach. This approach begins by looking at different sources of contaminants such as industries and power plants. Then, it is important to find out if the contaminant of interest has reached a receptor (usually humans). With this approach, it is very hard to prove that a pollutant from a source has reached a target.

Exposure

In this context exposure is defined as the contact between an agent and a target. Contact takes place at an exposure surface over an exposure period. Mathematically, exposure is defined as
E=t1t2C(t)dt

where E is exposure, C(t) is a concentration that varies with time between the beginning and end of exposure. It has dimensions of mass times time divided by volume. This quantity is related to the potential dose of contaminant by multiplying it by the relevant contact rate, such as breathing rate, food intake rate etc. The contact rate itself may be a function of time.[6]

Routes of exposure

Contact between a contaminant and an organism can occur through any route. The possible routes of exposure are:

  • Inhalation, if the contaminant is present in the air
  • Ingestion, through food, drinking or hand-to-mouth behavior
  • Dermal absorption, if the contaminant can be absorbed through the skin

Exposure to a contaminant can and does occur through multiple routes, simultaneously or at different times. In many cases the main route of exposure is not obvious and needs to be investigated carefully. For example, exposure to byproducts of water chlorination can obviously occur by drinking, but also through the skin, while swimming or washing, and even through inhalation from droplets aerosolized during a shower. The relative proportion of exposure from these different routes cannot be determined a priori.

Measurement of exposure

To quantify the exposure of particular individuals or populations two approaches are used, primarily based on practical considerations:

Direct approach

  • The direct approach measures the exposures to pollutants by monitoring the pollutant concentrations reaching the respondents. The pollutant concentrations are directly monitored on or within the person through point of contact, biological monitoring, or biomarkers. The point of contact approach indicates the total concentration reaching the host, while biological monitoring and the use of biomarkers infer the dosage of the pollutant through the determination of the body burden.[7] The respondents often record their daily activities and locations during the measurement of the pollutants to identify the potential sources, microenvironments, or human activities contributing the pollutant exposure.[8] An advantage of the direct approach is that the exposures through multiple media (air, soil, water, food, etc.) are accounted for through one study technique. The disadvantages include the invasive nature of the data collection and associated costs.
    • Point of contact - Continuous measure of the contaminant reaching the target through all routes.
    • Biological Monitoring - is another approach to measuring exposure.[9] This measures the amount of a pollutant within the body in various tissue media such adipose tissue, bone, or urine. Biological monitoring measures the body burden of a pollutant but not the source from whence it came. The substance measured may be either the contaminant itself or a biomarker which is specific to and indicative of an exposure to the contaminant.
    • Biomarkers of Exposure Assessment - Measure of the contaminant or other proportionally related variable in the body.

Indirect approach

  • The indirect approach measures the pollutant concentrations in various locations or during specific human activities to predict the exposure distributions within a population. The indirect approach focuses on the pollutant concentrations within microenvironments or activities rather than the concentrations directly reaching the respondents. The measured concentrations are correlated to large-scale activity pattern data, such as the National Human Activity Pattern Survey (NHAPS), to determine the predicted exposure by multiplying the pollutant concentrations by the time spent in each microenvironment or activity for by multiplying the pollutant concentrations b the contact rate with each media.[10] The indirect approach or exposure modeling determines the estimated exposure distributions within a population rather than the direct exposure an individual has experienced. The advantage is that process is minimally invasive to the population and is associated with lower costs than the direct approach. A disadvantage of the approach is that the results were determined independently of any actual exposures, so the exposure distribution is open to errors from any inaccuracies in the assumptions made during the study, the time-activity data, or the measured pollutant concentrations.

In general, direct methods tend to be more accurate but more costly in terms of resources and demands placed on the subject being measured and may not always be feasible, especially for a population exposure study. Examples of direct methods include air sampling though a personal portable pump, split food samples, hand rinses, breath samples or blood samples. Examples of indirect methods include environmental water, air, dust, soil or consumer product sampling coupled with information such as activity/location diaries. Mathematical exposure models may also be used to explore hypothetical situations of exposure.[11]

Exposure factors

Especially when determining the exposure of a population rather than individuals, indirect methods can often make use of relevant statistics about the activities that can lead to an exposure. These statistics are called exposure factors. They are generally drawn from the scientific literature or governmental statistics. For example they may report informations such as amount of different food eaten by specific populations, divided by location or age, breathing rates, time spent for different modes of commuting, showering or vacuuming, as well as information on types of residences. Such information can be combined with contaminant concentrations from ad-hoc studies or monitoring network to produce estimates of the exposure in the population of interest. These are especially useful in establishing protective standards.

Exposure factor values can be used to obtain a range of exposure estimates such as average, high-end and bounding estimates. For example, to calculate the lifetime average daily dose one would use the equation below:

LADD=(ContaminantConcentration)(IntakeRate)(ExposureDuration)/(BodyWeight)(AverageLifetime)

All of the variables in the above equation, with the exception of contaminant concentration, are considered exposure factors. Each of the exposure factors involves humans, either in terms of their characteristics (e.g., body weight) or behaviors (e.g., amount of time spent in a specific location, which affects exposure duration). These characteristics and behaviors can carry a great deal of variability and uncertainty. In the case of lifetime average daily dose, variability pertains to the distribution and range of LADDs amongst individuals in the population. The uncertainty, on the other hand, refers to exposure analyst's lack of knowledge of the standard deviation, mean, and general shape when dealing with calculating LADD.

The U.S. Environmental Protection Agency's Exposure Factors Handbook[12] provides solutions when confronting variability and reducing uncertainty. The general points are summarized below:

Four Strategies for Confronting Variability[13] Examples
Disaggregate the variability Develop distribution of body weight for subgroup
Ignore the variability Assume all adults weigh 65 kg
Use a maximum or minimum value Choose a high-end value for weight distribution
Use the average value Use the mean body weight for all adults
Analyzing Uncertainty[14] Description
Classical statistical methods (descriptive statistics and inferential statistics) Estimating the population exposure distribution directly, based on measured values from a representative sample
Sensitivity analysis Changing one input variable at a time while leaving others constant, to examine effect on output
Propagation of uncertainty Examining how uncertainty in individual parameters affects the overall uncertainty of the exposure assessment
Probabilistic analysis Varying each of the input variables over various values of their respective probability distributions(i.e. Monte Carlo integration)

Defining Acceptable Exposure for Occupational Environments

Simple representation of exposure risk assessment and management hierarchy based on available information

Occupational exposure limits are based on available toxicology and epidemiology data to protect nearly all workers over a working lifetime. Exposure assessments in occupational settings are most often performed by Occupational / Industrial Hygiene (OH/IH) professionals who gather "Basic Characterization" consisting of all relevant information and data related to workers, agents of concern, materials, equipment and available exposure controls. The exposure assessment is initiated by selecting the appropriate exposure limit averaging time and "decision statistic" for the agent. Typically the statistic for deciding acceptable exposure is chosen to be the majority (90%, 95% or 99%) of all exposures to be below the selected occupational exposure limit. For retrospective exposure assessments performed in occupational environments, the "decision statistic" is typically a central tendancy such as the mean or geometric mean or median for each worker or group of workers. Methods for performing occupational exposure assessments can be found in "A Strategy for Assessing and Managing Occupational Exposures, Third Edition Edited by Joselito S. Ignacio and William H. Bullock". [15]

Exposure assessment is a continuous process that is updated as new information and data becomes available.

See also

External links

  • National Academy of Sciences Report (2012), "Exposure Science in the 21st Century"
  • AIHA EASC AIHA Exposure Assessment Strategies Committee
  • AIHA - American Industrial Hygiene Association (AIHA)
  • ISES - International Society of Exposure Science
  • Exposure Factors Handbook by the US EPA
  • Software to Estimate Human Exposure to Chemicals by Creme Software
  • A Strategy for Assessing and Managing Occupational Exposures, Third Edition Edited by Joselito S. Ignacio and William H. Bullock AIHA Press
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References

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  1. US EPA, "Expocast" http://www.epa.gov/ncct/expocast/
  2. Vallero, D.A. (2004). Environmental Contaminants: Assessment and Control. Academic Press, New York, NY, ISBN 0127100571
  3. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12464 US EPA Exposure Factors Handbook
  4. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12464 US EPA Exposure Factors Handbook
  5. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12464 US EPA Exposure Factors Handbook
  6. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=15263 US EPA Guidelines for Exposure Assessment
  7. Ott, Wayne, Anne Steinemann, and Lance Wallace. Exposure Analysis. Taylor & Francis group, 2007. 8-13.
  8. Ott, Wayne, Anne Steinemann, and Lance Wallace. Exposure Analysis. Taylor & Francis group, 2007. 8-13.
  9. One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

    In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

    Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

    Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

    A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

    The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

    There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
  10. Ott, Wayne, Anne Steinemann, and Lance Wallace. Exposure Analysis. Taylor & Francis group, 2007. 8-13.
  11. Vallero D., Isukapalli S., Zartarian V., McCurdy T., McKone T., Georgopoulos P.G. and Dary C. (2010). Modeling and predicting pesticide exposures. Chapter 44. In Hayes’ Handbook of Pesticide Toxicology - Third Edition. Krieger, R. (ed.), Academic Press: pp. 995–1020. [1]
  12. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12464 US EPA Exposure Factors Handbook
  13. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12464 US EPA Exposure Factors Handbook
  14. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12464 US EPA Exposure Factors Handbook
  15. "A Strategy for Assessing and Managing Occupational Exposures, Third Edition Edited by Joselito S. Ignacio and William H. Bullock AIHA Press" http://iweb.aiha.org/iweb/Purchase/ProductDetail.aspx?Product_code=AEAK06-327/