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Toxicology

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How can I use NOAEC values?

The NOAEC is a parameter identical to the NOAEL except for the fact that a concentration is used instead of a dose (the most common case is when a gaseous substance is administered to the animal).
Most frequently, NOAECs are expressed as mg/m3, mg/L or ppm. Two parameters are needed in order to provide a conversion for the unit of measure:

  • The mean respiratory volume of the animal used for the test
  • The value of absorption of the inhaled substance
The latter parameter is fundamental, as the assumption of an absorption equal to 100% would lead to a big underestimation of the real toxicity (in case for example all the symptoms are caused by the 10% of the administered substance).
The NOAEL-project is capable of converting the NOAEC to NOAEL only if the dermal absorption is available; otherwise the substance will be classified as "No NOAEL found".
 

Can I derive a NOAEL from an ADI value?

The Acceptable Daily Intake (ADI) indicates the amount of substance that can be ingested (orally) on a daily basis over a lifetime without an appreciable health risk. This parameter is obtained dividing a NOAEL for a factor equal to 10. Since many ingredients such as preservatives are used both in food and cosmetics, toxicological data used to calculate the ADI are accepted for safety evaluation of cosmetics. However we can't derive a NOAEL simply dividing an ADI by a factor of 10 because crucial informations would be missing (e.g. animal model used to investigate toxicity, time and route of subministration etc...).

Can I derive the NOAEL from LD50?

Although some articles report that it is possible to extrapolate from LD50 to NOAEL1, this approach is totally wrong because of several reasons:
  • LD50 value is related to lethality (an endpoint that can't be classified as an adverse effect)
  • LD50 is calculated from acute exposure studies, whereas the NOAEL is extrapolated from longer term ones.
LD50 is used to classify toxic agents and a threshold for non-adverse effect is totally missing from the experimental design of employed tests.
In a typical LD50 protocol, a large amount of test-substance is used within 24 hour period, on the other hand repeated dose toxicity tests employ lower dose-range (which, in turn, are calculated from the outcome of 14-days repeated dose toxicity test and LD50).
 
Bibliography
 
1Pieters et al. Regulatory Toxicology and Pharmacology, Volume 23, Issue 3, June 1996, Pages 249–255

What is the correct definition for NOEL, NOAEL, and LOAEL?

All these acronyms are parameters employed in toxicology to determine the potential hazard of substances; they are the result of repeated dose toxicity studies carried on animal model systems and are fundamental to understand potential risks to humans exposed to the substance investigated.
Still today there are no consistent standard definitions for the terms NOEL, or NOAEL or the corresponding LOEL and LOAEL. 
The most complete and clear definition for NOAEL is “The highest exposure level at which there are no statistically or biologically significant increases in the frequency or severity of adverse effects between the exposed population and its appropriate control. Some effects may be produced at this level, but they are not considered to be adverse or precursors to adverse effects.”
The highest exposure level at which no effects (adverse or nonadverse) are observed in the exposed population is the NOEL. 
The LOAEL corresponds to the lowest exposure level at which there are statistically or biologically significant increases in the frequency or severity of adverse effects between the exposed population and its appropriate control.
 
It is obvious that, in order to distinguish NOAEL from NOEL, there is a need for definitions of “adverse” and “biologically significant”, especially considering that many organizations will actually be employing NOAELs in their regulatory process even though they may refer to them as a NOELs.
  • An adverse effect is defined as a biochemical, morphological or physiological change (in response to a stimulus) that either singly or in combination adversely affects the performance of the whole organism or reduces the organism’s ability to respond to an additional environmental challenge. 
  • A biologically significant effect is a response (to a stimulus) in an organism or other biological system that is considered to have substantial or noteworthy effect (positive or negative) on the well-being of the biological system. The concept is to be distinguished from statistically significant effects or changes, which may or may not be meaningful to the general state of health of the system.
 
Bibliography
 
Carpanini et al TOXICOLOGIC PATHOLOGY, vol 30, no 1, pp 66–74, 2002
 
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