Half-life is defined as the time required for half of the unstable nuclei to undergo their decay process. Each substance has a different half-life. For example, carbon-10 has a half-life of only 19 seconds, making it impossible for this isotope to be encountered in nature. Uranium-233, on the other hand, has the half-life of about 160 000 years. Directed by Dennis O'Rourke. A documentary on American nuclear testing in the Pacific atolls. The half-life of a drug is an estimate of the period of time that it takes for the concentration or amount in the body of that drug to be reduced by exactly one half (50%). The symbol for half-life is T½. For example, if 100mg of a drug with a half-life of 60 minutes is taken, the following is estimated. Half-Life Named Game of the Year by over 50 publications, Valve's debut title blends action and adventure with award-winning technology to create a frighteningly realistic world where players must think to survive. Also includes an exciting multiplayer mode that allows you to play against friends and enemies around the world. The popular game has got a flash version. Kill all the enemies with your weapon that has got 45 bullets per magazine.
What is a pesticide half-life?
A half-life is the time it takes for a certain amount of a pesticide to be reduced by half. This occurs as it dissipates or breaks down 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 remaining is nearly zero. See Figure 1.
Figure 1. Approximate amount of pesticide (shaded area) remaining at the application site over time.
Each pesticide can have many half-lives depending on conditions in the environment. For example, permethrin breaks down at different speeds in soil, in water, on plants, and in homes.
- In soil, the half-life of permethrin is about 40 days, ranging from 11-113 days.
- In the water column, the half-life of permethrin is 19-27 hours. If it sticks to sediment, it can last over a year.
- On plant surfaces, the half-life of permethrin ranges from 1-3 weeks, depending on the plant species.
- Indoors, the half-life of permethrin can be highly variable. It is expected to be over, or well over, 20 days.
Why is a pesticide's environmental half-life important?
The half-life can help estimate whether or not a pesticide tends to build up in the environment. Pesticide half-lives can be lumped into three groups in order to estimate persistence. These are low (less than 16 day half-life), moderate (16 to 59 days), and high (over 60 days). Pesticides with shorter half-lives tend to build up less because they are much less likely to persist in the environment. In contrast, pesticides with longer half-lives are more likely to build up after repeated applications. 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 to control aphids in the garden for several weeks. One application of a pesticide with a half-life of a few hours will probably not be very effective several weeks out. This is because the product would have broken down to near-zero amounts after only a few days. This type of product would likely have to be applied multiple times over those several weeks. This could increase the risk of exposure to people, non-target animals, and plants.
What can influence a pesticide's environmental half-life?
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, the properties of the formulation may dominate initially, until enough time has passed to allow the ingredients to separate This is because small amounts of an active ingredient are 'formulated' with larger amounts of 'other' ingredients to make a whole pesticide product.
Table 1. Environmental factors that affect pesticide persistence.4
Half-life Definition
| Environmental Factors | Role in Chemical Degradation |
|---|---|
| Sunlight | Radiation from the sun breaks certain chemical bonds, creating break down products. |
| Microbes | Bacteria and fungi can break down chemicals, creating biodegradation products. |
| Plant / Animal Metabolism | Plants and animals can change chemicals into forms that dissolve better in water (metabolites). This makes removal from the body easier. |
| Water | Water breaks chemicals apart to make pieces that dissolve better in water (hydrolysis). This is typically a very slow process. |
| Dissociation | Chemicals can break apart into smaller pieces (dissociation products). |
| Sorption | Chemicals that stick tightly to particles can become inaccessible and/or move away with those particles. |
| Bioaccumulation | Some chemicals can be absorbed by plants/animals from the soil, water, food, and air. When the plant/animal is exposed again before it can remove the chemical(s), accumulation can occur. |
How is a pesticide's half-life determined?
Pesticide half-lives are often determined in a laboratory. There, conditions like temperature can be controlled and closely monitored. Soil, water, or plant material is mixed with a known amount of a pesticide. The material is then sampled and tested 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 plant material. It is then placed in an outdoor environment where it is exposed to various environmental conditions and tested over 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 a variety of databases, books, and peer-reviewed articles. If you need help, call the National Pesticide Information Center.
What happens to pesticides after they 'go away'?
When a pesticide breaks down it doesn't disappear. Instead, it forms new chemicals that may be more or less toxic than the original chemical. Generally, they are broken into smaller and smaller pieces until only carbon dioxide, water, and minerals are 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.
Inorganic pesticides like iron phosphate and copper sulfate don't break down in the same way as organic pesticides.10,12 The 'half-life' concept only applies to organic pesticides, those that contain carbon components.
Figure 2. The soil half-life of five pesticides.8,9,11,13,15
Where can I get more information?
For more detailed information about pesticide half-lives please visit the list of referenced resources below or call the 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 http://npic.orst.edu. NPIC provides objective, science-based answers to questions about pesticides.
Please cite as: Hanson, B.; Bond, C.; Buhl, K.; Stone, D. 2015. Pesticide Half-life Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/half-life.html.
The half-life of a reaction, t1/2, is the amount of time needed for a reactant concentration to decrease by half compared to its initial concentration. Its application is used in chemistry and medicine to predict the concentration of a substance over time. The concepts of half life plays a key role in the administration of drugs into the target, especially in the elimination phase, where half life is used to determine how quickly a drug decrease in the target after it has been absorbed in the unit of time (sec, minute, day,etc.) or elimination rate constant ke (minute-1, hour-1, day-1,etc.). It is important to note that the half-life is varied between different type of reactions. The following section will go over different type of reaction, as well as how its half-life reaction are derived. The last section will talk about the application of half-life in the elimination phase of pharmcokinetics.
Half-life Definition
Zero order kinetics
In zero-order kinetics, the rate of a reaction does not depend on the substrate concentration. In other words, saturating the amount of substrate does not speed up the rate of the reaction. Below is a graph of time (t) vs. concentration [A] in a zero order reaction, several observation can be made: the slope of this plot is a straight line with negative slope equal negative k, the half-life of zero order reaction decrease as the concentration decrease.
We learn that the zero order kinetic rate law is as followed, where [A] is the current concentration, [A]0 is the initial concentration, and k is the reaction constant and t is time:
[ [A]= [A]_0 - kt label{1}]
In order to find the half life we need to isolate t on its own, and divide it by 2. We would end up with a formula as such depict how long it takes for the initial concentration to dwindle by half:
[t_{1/2} = dfrac{[A]_0}{2k} label{2}]
The t1/2 formula for a zero order reaction suggests the half-life depends on the amount of initial concentration and rate constant.
First Order Kinetics
In First order reactions, the graph represents the half-life is different from zero order reaction in a way that the slope continually decreases as time progresses until it reaches zero. We can also easily see that the length of half-life will be constant, independent of concentration. For example, it takes the same amount of time for the concentration to decrease from one point to another point.
In order to solve the half life of first order reactions, we recall that the rate law of a first order reaction was:
[[A]=[A]_0e^{-kt} label{4}]
To find the half life we need to isolate t and substitute [A] with [A]0/2, we end up with an equation looking like this:
[t_{1/2} = dfrac{ln 2}{k} approx dfrac{0.693}{k} label{5}]
The formula for t1/2 shows that for first order reactions, the half-life depends solely on the reaction rate constant, k. We can visually see this on the graph for first order reactions when we note that the amount of time between one half life and the next are the same. Another way to see it is that the half life of a first order reaction is independent of its initial concentration.
Second Order Reactions
Half-life of second order reactions shows concentration [A] vs. time (t), which is similar to first order plots in that their slopes decrease to zero with time. However, second order reactions decrease at a much faster rate as the graph shows. We can also note that the length of half-life increase while the concentration of substrate constantly decreases, unlike zero and first order reaction.
In order to solve for half life of second order reactions we need to remember that the rate law of a second order reaction is:
[dfrac{1}{[A]} = kt + dfrac{1}{[A]_0} label{6}]
As in zero and first order reactions, we need to isolate T on its own:
[t_{1/2} = dfrac{1}{k[A]_0} label{7}]
This replacement represents half the initial concentration at time, t (depicted as t1/2). We then insert the variables into the formula and solve for t1/2. The formula for t1/2 shows that for second order reactions, the half-life only depends on the initial concentration and the rate constant.
Example 1: Pharmacokinetics
A following example is given below to illustrate the role of half life in pharmacokinetics to determine the drugs dosage interval.
The therapeutic range of drug A is 20-30 mg/L. Its half life in the target in 5 hours. Once the drug is metabolized in the target, its concentration will decrease over time. To ensure its maximal effect of the drug in the target, the administration will be monitored so that the minimum serum concentration will never go lower than 20 mg/L and the maximum serum concentration will never exceed 30mg/L. As a result, it is important to administer drug A to the target every 5 hours to ensure its effective therapeutic range.
Another important application of half life in pharmacokinetics is that half-life tells how tightly drugs bind to each ligands before it is undergoing decay (ks). The smaller the value of ks, the higher the affinity binding of drug to its target ligand, which is an important aspect of drug design
Problems
1. Define the following term: therapeutic range, half-life, zero order reaction, first order reaction, second order reaction.
2. Examine the following graph and answer
What is the therapeutic range of drug B?
From the graph, estimate the dosage interval of drug B to ensure its maximum effect?
The patient forgot to take the drug at the end of the dosage interval, he decided to take double the amount of drug B at the end of the next dosage interval. Will the drug still be in its therapeutic range?
3. A patient is treating with 32P. How long does it takes for the radioactivity to decay by 90%? The half-life of the material is 15 days.
4. In first order half life, what is the best way to determine the rate constant k? Why?
5. In a first order reaction, A --> B. The half-life is 10 days.
Determine its rate constant k?
Tai Half-life 1.1
How much time required for this reaction to be at least 50% and 60% complete?
Solutions
1. Therapeutic range: the range that is between maximum drug concentration and a minimum drug concentration in which it is capable of fully exhibit its effective activity
Half-life: the amount of time it required for a reaction to undergoing decay by half.
Zero order reaction, First order reaction, Second order reaction : see module to understand its definition
2. Looking at the graph, we can see the therapeutic range is the amplitude of the graph, which is 5-15 mg/L
The dosage interval is the half-life of the drug, looking at the graph, the half-life is 10 hours.
Even though it will get in the therapeutic range, such practice is not recommended.
3. Using k=ln2/t1/2, plug in half-life we will find k = 4.62x10-2 day-1
If we want the product to decay by 90%, that means 10% is left non-decayed, so [A]t/[A]o = .1
From ln([A]t/[A]o) = -kt, plug in value of k and [A]t/[A]o we then have t = 50 days
4. The best way to determine rate constant k in half-life of first order is to determine half-life by experimental data. The reason is half-life in first order order doesnt depend on initial concentration.
5. Rate constant, k, will be equal to k=ln2/t1/2 , so k = 0.0693 day-1
For the reaction to be 50% complete, that will be exactly the half-life of the reaction at 10 days
For the reaction to be 60% complete, using the similar equation derived from question 4, we have [A]t/[A]o = .4 --> t = 13.2 days
References
- Chang, Raymond. Physical Chemistry for the Biosciences. Sausalito,CA: University Science Books. Pages 312-319. 2005
- Bauer, Larry. Applied Clinical Pharmacokinetics. New York City, New York: McGraw-Hill. 2008
- Mozayani, Ashraf and Raymond, Lionel. Handbook of Drug Interactions: A Clinical and Forensic Guide. Totowa, New Jersy: Humana Press. 2003
Contributors and Attributions
- David Macias, Samuel Fu, Sinh Le