Half Life 1 Map List

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Ahalf-life is the time it takes for a certain amount of a pesticide to be reduced by half. This occurs as it dissipates or breaksdown in the environment. In general, a pesticide will break down to 50% of the original amount after a single half-life.After two half-lives, 25% will remain. About 12% will remain after three half-lives. This continues until the amount remainingis nearly zero. See Figure 1.

The half-life can help estimate whether or not a pesticide tends to build up in the environment. Pesticide half-lives canbe lumped into three groups in order to estimate persistence. These are low (less than 16 day half-life), moderate (16to 59 days), and high (over 60 days). Pesticides with shorter half-lives tend to build up less because they are much lesslikely to persist in the environment. In contrast, pesticides with longer half-lives are more likely to build up after repeatedapplications. This may increase the risk of contaminating nearby surface water, ground water, plants, and animals.

However, pesticides with very short half-lives can have their drawbacks. For example, imagine that a pesticide is needed tocontrol aphids in the garden for several weeks. One application of a pesticide with a half-life of a few hours will probablynot be very effective several weeks out. This is because the product would have broken down to near-zero amounts afteronly a few days. This type of product would likely have to be applied multiple times over those several weeks. This couldincrease the risk of exposure to people, non-target animals, and plants.

Many things play a role in how long a pesticide remains in the environment. These include things like sunlight, temperature,the presence of oxygen, soil type (sand, clay, etc.), how acidic the soil or water is, and microbe activity. See Table 1.Pesticide half-lives are commonly reported as time ranges. This is because environmental conditions can change over time.This makes it impossible to describe a single, consistent half-life for a pesticide.

A pesticide product's formulation can also change how the active ingredient behaves in the environment. In fact, theproperties of the formulation may dominate initially, until enough time has passed to allow the ingredients to separateThis is because small amounts of an active ingredient are 'formulated' with larger amounts of 'other' ingredients to make awhole pesticide product.

Pesticide half-lives are often determined in a laboratory. There, conditions like temperature can be controlled and closelymonitored. Soil, water, or plant material is mixed with a known amount of a pesticide. The material is then sampled andtested over time to determine how long it takes for half of the chemical to break down.

Field studies are also performed for some chemicals. A known amount of the pesticide is mixed with soil, water, or plantmaterial. It is then placed in an outdoor environment where it is exposed to various environmental conditions and testedover time. Field studies provide researchers with a more realistic idea of how the pesticide will act in the environment.However, half-life values from such studies can vary greatly depending on the exact conditions. See Figure 2.

Before a pesticide product is registered, manufacturers measure their half-lives. You can find their research results in avariety of databases, books, and peer-reviewed articles. If you need help, call the National Pesticide Information Center.

When a pesticide breaks down it doesn't disappear. Instead, it forms new chemicals that may be more or less toxic than theoriginal chemical. Generally, they are broken into smaller and smaller pieces until only carbon dioxide, water, and mineralsare left. Microbes often play a large role in this process. In addition, some chemicals may not break down initially. Instead,they might move away from their original location. It all depends on the chemical and the environmental conditions.

Certain pesticides like iron phosphate and copper sulfate don't break down in the same way as others, because they are based on chemical elements.10,12 The half-life concept only applies to pesticides with molecular structures that include carbon atoms.

For more detailed information about pesticide half-lives please visit the list of referenced resources below or callthe National Pesticide Information Center, Monday - Friday, between 8:00am - 12:00pm Pacific Time (11:00am - 3:00pm Eastern Time) at 1-800-858-7378 or visit us on the web at NPIC provides objective, science-based answers to questions about pesticides.

NPIC fact sheets are designed to answer questions that are commonlyasked by the general public about pesticides that are regulated by theU.S. Environmental Protection Agency (U.S. EPA). This document isintended to be educational in nature and helpful to consumers formaking decisions about pesticide use.

Half-life in the context of medical science typically refers to the elimination half-life. The definition of elimination half-life is the length of time required for the concentration of a particular substance (typically a drug) to decrease to half of its starting dose in the body. Understanding the concept of half-life is useful for determining excretion rates as well as steady-state concentrations for any specific drug. Different drugs have different half-lives; however, they all follow this rule: after one half-life has passed, 50% of the initial drug amount is removed from the body. The characteristic decreases of drugs over time have long been studied in a field known as pharmacokinetics and are depictable by basic differential equations. Most clinically relevant drugs tend to follow first-order pharmacokinetics; that is, their drug-elimination rates are proportional to plasma concentrations.[1] In contrast, a few drugs follow zero-order elimination in which the drug amount decreases by a constant amount over time regardless of initial concentration (i.e., ethanol). This article will focus on first-order half-life elimination as it is the most frequently encountered in clinical practice.

Half-life elimination is graphically represented with elimination curves that track the amount of a drug in the body over time, typically with time on the independent axis and drug plasma concentration on the dependent axis, as shown in Figure 1. Total drug exposure over time is represented in these graphs as the integral area under the curve (AUC).[2] Elimination curves are useful for determining if a drug indeed follows first-order kinetics, in which case the curve should follow a logarithmic decay according to the integrated rate law of first-order reactions (Equation 1). After solving the differential equation, one can obtain the half-life equation that is commonly tested on and used in clinical practice (Equation 2). From this equation, one can quickly determine the half-life of a drug, given its predetermined rate constant k. An alternative half-life equation exists that relates half-life to other pharmacokinetic parameters known as the volume of distribution and clearance (Equation 3).[3][4]

It is also worth discussing the relationship between the percentage of drug eliminated and the number of half-lives. Assuming no administration of additional drug after an initial dose, ignoring any drug-drug interactions, and assuming a physiologically healthy individual, certain quantitative constants apply to all drugs exhibiting first-order pharmacokinetics. For example, 90% of a given drug will have undergone elimination after approximately 3.3 half-lives. Even further, 94 to 97% of a drug will have been eliminated after 4 to 5 half-lives. Thus, it follows that after 4 to 5 half-lives, the plasma concentrations of a given drug will be below a clinically relevant concentration and thus will be considered eliminated. Conversely, the accumulation of a drug can reach a steady-state during an infusion. When administering a drug at regular intervals or a constant amount (such as an infusion), the drug achieves a given steady-state concentration after approximately 4 to 5 half-lives without any further accumulation in the body with repeated doses.[5] This state is because the infusion rate and the clearance of the drug will have reached an equilibrium, and thus the net concentration of drug in the body will remain constant. The value of this steady-state concentration is determined by the dosage, dosing interval, and clearance.

Half-life is one of the oldest pharmacokinetic parameters discussed in the medical community yet continues to be a source of confusion for many medical students and even well-versed clinicians.[6] For this reason, the USMLE examiners continue to evaluate medical students and licensed physicians on this elusive topic. Within the concept of half-life, many assumptions are necessary, including a one-compartment system metabolizing the drug, a perfectly first-ordered system free of any renal or hepatic deficiencies, and an isolated system without any drug-drug interactions or alternative metabolic pathways. This situation is seldom the case in a clinical setting where physicians have patients who present with chronic kidney disease or other ailments, and who may take numerous medications with potential drug interactions. Also, patient age is a significant factor in determining the accurate half-life of a drug, particularly for pediatric and geriatric patients in which drug metabolism and thus half-life can vary significantly from a healthy middle-aged adult. Because of the highly theoretical model of half-life, it is often challenging to implement into practice and use it as a tool for clinical decision making. Thus, medical students and physicians need to factor such realities into half-life calculations for effective and safe pharmacological management. Numerous studies have attempted to establish methodologies that account for such nuances in the management of disease based on individual pharmacokinetic drug profiles.[7][8]

The clinical significance of half-life tends to arise in situations involving drug toxicity. These incidences can result from patients who have overdosed or received an incorrect amount of a particular drug from medical staff, who have clinically significant renal or hepatic failure, or who have any other multitude of factors that increase drug plasma concentrations above a given toxic threshold. In the case of renal failure, drug excretion will be impaired, and consequently, the peak initial concentration and excretion rate of a given drug will increase.[9] Hepatic disease also affects the half-life of a given drug due to impaired metabolism. Because the liver inactivates active metabolites at a slower rate, the body will take a more extended period to remove the drug from circulation.[10] Half-life is also clinically relevant when physicians must determine the most efficient yet safest dosing schedule to achieve an optimal therapeutic effect, or when a steady-state concentration of a drug is desirable. The regular occurrence of these types of clinical scenarios explains why medical professionals rely so frequently on half-life drug calculations in practice, and why it continues to receive emphasis throughout medical education.

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