Hazard Assessment and Contaminated Sites - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 565

stimulus which comes just within the limits of perception, and below which level a
recognisable response is not elicited.
TDI Tolerable Daily Intake. An estimate of the intake of a contaminant which can
occur over a lifetime without appreciable health risk. It may have different units
depending on the route of administration (WHO 1994 ). (Imray and Langley 1996 ,
p. 18). The term, “acceptable” daily intake is used for contaminants such as pes-
ticides (herbicides, insecticides, antifungals etc.) which are deliberately used on
food crops or food-producing animals and for which some level of residues may be
expected to occur in food. The term “tolerable” daily intake is used when the con-
taminant is a potential food or environmental contaminant. Whilst exposure should
not occur, a TDI is an established health limit below which lifetime exposure should
not have any adverse health effects.“Acceptable Daily Intake” and “Reference
Dose”.
TWI Tolerable Weekly Intake. The TI expressed as a weekly amount
Tolerable Intake “an estimate of the intake of a contaminant that over a lifetime
is without appreciable health risk.” (WHO 1994 ). Examples are the ADI, TDI and
Reference Dose.
Toxicity The quality or degree of being poisonous or harmful to plant, animal or
human life.
Tumour A mass of abnormal, disorganised cells, arising from pre-existing tissue,
which is characterised by excessive and uncoordinated cell proliferation or growth
and by abnormal differentiation (specialisation). There are two types of tumours,
benign and malignant. Benign tumours morphologically resemble their tissue of
origin, grow slowly (may also stop growing) and form encapsulated masses; they
do not infiltrate other tissues, they do not metastasise and are rarely fatal. Malignant
tumours resemble their parent tissue less closely and are composed of increasingly
abnormal cells genetically, morphologically and functionally. Most grow rapidly,
spread progressively through adjacent tissues and metastasise to distant tissues.
Uncertainty The lack of knowledge about the correct value, e.g. a specific
exposure measure or estimate
Uncertainty factor A numerical factor applied to the no-effect level to derive
an exposure level considered to be without appreciable risk to health (the NEL is
divided by the uncertainty factor). The magnitude of the uncertainty factor depends
on the nature of the toxicity observed, the quality of the toxicological data available,
and whether the effects were observed in humans or animals (IEH 1999 ).
Variability Measurable factors that differ e.g. height is variable across popula-
tions. The major types of variability are temporal, spatial and interindividual. They
may be discrete (e.g. albinism) or continuous (e.g., body weight). It may be read-
ily identifiable (e.g., presence of albinism) or difficult to identify (e.g., ability to
detoxify a particular contaminant metabolite)