Reference dose
Encyclopedia
A reference dose is the United States Environmental Protection Agency
's maximum acceptable oral dose
of a toxic substance. Reference doses are most commonly determined for pesticide
s. The EPA defines an oral reference dose (abbreviated RfD) as:
, RfDs are not enforceable standards. Instead, the EPA uses RfDs as risk assessment
benchmarks and tries to set other regulations so that people are not exposed to chemicals in amounts that exceed RfDs. According to the EPA, "[a]n aggregate daily exposure to a [chemical] at or below the RfD (expressed as 100 percent or less of the RfD) is generally considered acceptable by EPA." For example, the EPA set an acute RfD for children of 0.0015 mg/kg/day for the organochlorine insecticide
endosulfan
, based on neurological effects observed in test animals. The EPA then looked at dietary exposure to endosulfan, and found that for the most exposed 0.1% of children age 1-6, their daily consumption of the endosulfan exceeded this RfD. To remedy this, the EPA revoked the use of endosulfan on the crops that contributed the most to exposure of children: certain beans, peas, spinach, and grapes.
RfDs are determined for the same substance. Reference doses are specific to dietary exposure. When assessing inhalation exposure, EPA uses "Reference Concentrations," (RfCs), instead of RfDs. Note that RfDs apply only to non-cancer effects. When evaluating carcinogen
ic effects, the EPA uses the Q1* method.
) are dosed with varying amounts of the substance in question, and the largest dose at which no effects are observed is identified. This dose level is called the "No observable effect level," or NOEL. To account for the fact that humans may be more or less sensitive than the test animal, a 10-fold uncertainty factor is usually applied to the NOEL. This uncertainty factor is called the "interspecies uncertainty factor" or Ufinter. An additional 10-fold uncertainty factor, the "intraspecies uncertainty factor" or Ufintra, is usually applied to account for the fact that some humans may be substantially more sensitive to the effects of substances than others. Additional uncertainty factors may also be applied. In general:
Frequently, a "No observable adverse effect level
" or NOAEL is used in place of a NOEL. If adverse effects are observed at all dose levels tested, then the smallest dose tested, the "Lowest observed adverse effect level" or LOAEL, is used to calculate the RfD. An additional uncertainty factor usually applied in these cases, since the NOAEL, by definition, would be lower than the LOAEL had it been observed. If studies using human subjects are used to determine a RfD, then the interspecies uncertainty factor can be reduced to 1, but generally the 10-fold intraspecies uncertainty factor is retained. Such studies are rare.
, adapted from the EPA's Interim Reregistration Eligibility Decision for chlorpyrifos.
The EPA determined the acute RfD to be 0.005 mg/kg/day based on a study in which male rats were administered a one time dose of chlorpyrifos and blood cholinesterase
activity was monitored. Cholinesterase inhibition was observed at all dose levels tested, the lowest of which was 1.5 mg/kg. This level was thus identified at the lowest observed adverse effect level (LOAEL). A NOAEL of 0.5 mg/kg was estimated by dividing the LOAEL by a 3-fold uncertainty factor. The NOAEL was then divided by the standard 10-fold inter- and 10-fold intraspecies unscertainty factors to arrive at the RfD of 0.005 mg/kg/day. Other studies showed that fetuses and children are even more sensitive to chlorpyrifos than adults, so the EPA applies an additional 10-fold uncertainty factor to protect this subpopulation. A RfD that has been divided by an additional uncertainty factor that only applies to certain populations is called a "population adjusted dose" or PAD. For chlorpyrifos, the acute PAD (or "aPAD") is thus 5*10-4 mg/kg/day, and it applies to infants, children, and women who are breast feeding.
The EPA also determined a chronic RfD for chlorpyrifos exposure based on studies in which animals were administered low doses of the pesticide for 2 years. Cholinesterase inhibition was observed at all dose levels tested, and a NOAEL of 0.03 mg/kg/day estimated by dividing a LOAEL of 0.3 mg/kg/day by an uncertainty facor of 10. As with the acute RfD, the chronic RfD of 3*10-4 mg/kg/day was determined by dividing this NOAEL by the inter- and intraspecies uncertainty factors. The chronic PAD ("cPAD") of 3*10-5 mg/kg/day was determined by applying an additional 10-fold uncertainty factor to account for the increased susceptibility of infants and children. Like the aPAD, this cPAD applies to infants, children, and breast feeding women.
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
's maximum acceptable oral dose
Dose (biochemistry)
A dose is a quantity of something 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...
of a toxic substance. Reference doses are most commonly determined for pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...
s. The EPA defines an oral reference dose (abbreviated RfD) as:
[A]n estimate, with uncertainty spanning perhaps an order of magnitudeOrder of magnitudeAn order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
, of a daily oral exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime.
Regulatory Status
Unlike National Ambient Air Quality StandardsNational Ambient Air Quality Standards
The National Ambient Air Quality Standards are standards established by the United States Environmental Protection Agency under authority of the Clean Air Act that apply for outdoor air throughout the country...
, RfDs are not enforceable standards. Instead, the EPA uses RfDs as risk assessment
Risk assessment
Risk assessment is a step in a risk management procedure. Risk assessment is the determination of quantitative or qualitative value of risk related to a concrete situation and a recognized threat...
benchmarks and tries to set other regulations so that people are not exposed to chemicals in amounts that exceed RfDs. According to the EPA, "[a]n aggregate daily exposure to a [chemical] at or below the RfD (expressed as 100 percent or less of the RfD) is generally considered acceptable by EPA." For example, the EPA set an acute RfD for children of 0.0015 mg/kg/day for the organochlorine insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...
endosulfan
Endosulfan
Endosulfan is an off-patent organochlorine insecticide and acaricide that is being phased out globally. Endosulfan became a highly controversial agrichemical due to its acute toxicity, potential for bioaccumulation, and role as an endocrine disruptor...
, based on neurological effects observed in test animals. The EPA then looked at dietary exposure to endosulfan, and found that for the most exposed 0.1% of children age 1-6, their daily consumption of the endosulfan exceeded this RfD. To remedy this, the EPA revoked the use of endosulfan on the crops that contributed the most to exposure of children: certain beans, peas, spinach, and grapes.
Types of RfDs
Reference doses are chemical specific, i.e. the EPA determines a unique reference dose for every substance it evaluates. Often separate acute and chronicChronic (medicine)
A chronic disease is a disease or other human health condition that is persistent or long-lasting in nature. The term chronic is usually applied when the course of the disease lasts for more than three months. Common chronic diseases include asthma, cancer, diabetes and HIV/AIDS.In medicine, the...
RfDs are determined for the same substance. Reference doses are specific to dietary exposure. When assessing inhalation exposure, EPA uses "Reference Concentrations," (RfCs), instead of RfDs. Note that RfDs apply only to non-cancer effects. When evaluating carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...
ic effects, the EPA uses the Q1* method.
Determination
RfDs are usually derived from animal studies. Animals (typically ratsRATS
RATS may refer to:* RATS , Regression Analysis of Time Series, a statistical package* Rough Auditing Tool for Security, a computer program...
) are dosed with varying amounts of the substance in question, and the largest dose at which no effects are observed is identified. This dose level is called the "No observable effect level," or NOEL. To account for the fact that humans may be more or less sensitive than the test animal, a 10-fold uncertainty factor is usually applied to the NOEL. This uncertainty factor is called the "interspecies uncertainty factor" or Ufinter. An additional 10-fold uncertainty factor, the "intraspecies uncertainty factor" or Ufintra, is usually applied to account for the fact that some humans may be substantially more sensitive to the effects of substances than others. Additional uncertainty factors may also be applied. In general:
Frequently, a "No observable adverse effect level
No observable adverse effect level (NOAEL)
The no observable adverse effect level denotes the level of exposure of an organism, found by experiment or observation, at which there is no biologically or statistically significant The no observable adverse effect level (NOAEL) denotes the level of exposure of an organism, found by experiment...
" or NOAEL is used in place of a NOEL. If adverse effects are observed at all dose levels tested, then the smallest dose tested, the "Lowest observed adverse effect level" or LOAEL, is used to calculate the RfD. An additional uncertainty factor usually applied in these cases, since the NOAEL, by definition, would be lower than the LOAEL had it been observed. If studies using human subjects are used to determine a RfD, then the interspecies uncertainty factor can be reduced to 1, but generally the 10-fold intraspecies uncertainty factor is retained. Such studies are rare.
An Example
As an example, consider the following determination of the RfD for the insecticide chlorpyrifosChlorpyrifos
Chlorpyrifos is a crystalline organophosphate insecticide that inhibits acetylcholinesterase and is used to control insect pests. It is known by many trade names...
, adapted from the EPA's Interim Reregistration Eligibility Decision for chlorpyrifos.
The EPA determined the acute RfD to be 0.005 mg/kg/day based on a study in which male rats were administered a one time dose of chlorpyrifos and blood cholinesterase
Cholinesterase
In biochemistry, cholinesterase is a family of enzymes that catalyze the hydrolysis of the neurotransmitter acetylcholine into choline and acetic acid, a reaction necessary to allow a cholinergic neuron to return to its resting state after activation.-Types:...
activity was monitored. Cholinesterase inhibition was observed at all dose levels tested, the lowest of which was 1.5 mg/kg. This level was thus identified at the lowest observed adverse effect level (LOAEL). A NOAEL of 0.5 mg/kg was estimated by dividing the LOAEL by a 3-fold uncertainty factor. The NOAEL was then divided by the standard 10-fold inter- and 10-fold intraspecies unscertainty factors to arrive at the RfD of 0.005 mg/kg/day. Other studies showed that fetuses and children are even more sensitive to chlorpyrifos than adults, so the EPA applies an additional 10-fold uncertainty factor to protect this subpopulation. A RfD that has been divided by an additional uncertainty factor that only applies to certain populations is called a "population adjusted dose" or PAD. For chlorpyrifos, the acute PAD (or "aPAD") is thus 5*10-4 mg/kg/day, and it applies to infants, children, and women who are breast feeding.
The EPA also determined a chronic RfD for chlorpyrifos exposure based on studies in which animals were administered low doses of the pesticide for 2 years. Cholinesterase inhibition was observed at all dose levels tested, and a NOAEL of 0.03 mg/kg/day estimated by dividing a LOAEL of 0.3 mg/kg/day by an uncertainty facor of 10. As with the acute RfD, the chronic RfD of 3*10-4 mg/kg/day was determined by dividing this NOAEL by the inter- and intraspecies uncertainty factors. The chronic PAD ("cPAD") of 3*10-5 mg/kg/day was determined by applying an additional 10-fold uncertainty factor to account for the increased susceptibility of infants and children. Like the aPAD, this cPAD applies to infants, children, and breast feeding women.