Dose (biochemistry)
A dose is a quantity of something (chemical, physical, or biological) that may impact an organism biologically; the greater the quantity, the larger the dose.  In nutrition, the term is usually applied to how much of a specific nutrient is in a person's diet or in a particular food, meal, or dietary supplement. In medicine, the term is usually applied to the quantity of a drug or other agent administered for therapeutic purposes. In toxicology dose may refer to the amount of a harmful agent (such as a poison, carcinogen, mutagen, or teratogen), to which an organism is exposed.
Chemicals are the most common things for which doses are measured, but there are others, such as radiation exposure.  For humans, most doses of micronutrients and medications are measured in milligrams (mg.), but some are measured in micrograms because of their potency.  Nonmedicinal poisons span the measurement scale; some poisons are so dangerous that a single microgram of it could be deadly, while other substances take much more.  For example, even water is toxic when consumed in large enough quantities.

# Effects are dose dependent
Dosage (the size of each dose) determines the strength and duration of the health benefits of nutrients, and also of the therapeutic effects of medical treatments.  Dosage also determines the severity of adverse effects of treatments and toxins.
Duration of  exposure, that is, the period of time over which the dose was received (all at once or gradually) also determines its effects (the body may build tolerance to gradual exposure to a drug, while a large immediate dose could be deadly).
The route by which a dose is exposed to, may affect the outcome, because some medications have different effects depending on whether they are inhaled, ingested, taken transdermally, injected, or inserted.
The dosage, route, concentration, and division over time may all be critical considerations in the administering of drugs, or in responding to exposure to a toxin.  In nutrition, the route is usually a given, as nutrients are generally eaten; while dosage and the frequency of ingestion of nutrients are very important variables in preventing disease and promoting overall health.

# Biological agents
Biological agents (bacteria, viruses, parasites) may have different dosage units.  This is because it is the ability of the organism to cause effects that is the important unit, not a specific quantity by weight, volume or even numerical count.  Often the unit used is CFU (colony forming units), which is proportionate to the number of organisms present multiplied times the number able to reproduce on a culture medium such as a Petri dish.

# Toxicology
In the realm of toxicology, several measures are commonly used to describe toxic dosages according to the degree of effect on an organism or a population, and some are specifically defined by various laws or organizational usage.  These include:
- LD50 = Median lethal dose, a dose which will kill 50% of an exposed population;
- NOEL = No Observed Effect Level, the highest dose known to show no effect;
- NOAEL = No Observed Adverse Effect Level, the highest dose known to show no adverse effects;
- PEL = Personal Exposure Limit, the highest concentration permitted under US OSHA regulations;
- STEL = Short Term Exposure Limit, the highest concentration permitted for short periods of time, generally 15-30 minutes;
- TWA = Time Weighted Average, the average amount of an agent's concentration over a specified period of time, usually 8 hours;
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