TCDD

Last updated: February 29. 2016


I. Background

Production.  TCDD, a polychlorinated dibenzodioxin (PCDD), enters the environment as a byproduct of industrial processes, such as waste incineration (medical and municipal), paper bleaching, and herbicide production.  Additionally, TCDD can enter the environment naturally from forest fires and volcanic eruptions.

Distribution in environment.  TCDD and other PCDDs enter the environment as mixtures and are associated with small particles in the air, especially during combustion.  Compounds associated with smaller particles have greater potential to travel further distances in the air.  If TCDD is released into waste water, some of it will be broken down by sunlight while the majority of it becomes deposited on the soil and settles to the bottom since TCDD is not water soluble.

Human exposure.  This persistent environmental contaminant is found at low levels in the environment, and people are exposed to small amounts of it each day primarily via the diet.  In fact, more than 90% of daily dioxin intake is through dairy products, meat, and fish.  Levels in the serum and adipose tissue of the general population typically range from 3-7 ppt, with the higher concentrations being found in adipose.  Inhalation and dermal exposures are rare.

Toxicity and perceived toxicity to humans/animals.  Little is known about the breakdown of TCDD in the human body, but its toxicity has been extensively studied in animals.  Many adverse effects, such as reproductive damage and birth defects, chloracne with acute exposure, weight loss, immune suppression, and carcinogenesis, have been reported following exposure; however, these animal studies utilize doses that are 10 to 1000 times the level typically found in people. 

II. MSDS Information

Structure of TCDD.  In its crystalline form, TCDD is colorless to white.

Vehicle to dilute TCDD.  First, crystalline TCDD is diluted in anisole at 1mg/ml then further into peanut oil such that the concentration brought into LAR is less than or equal to 1.5 µg/ml TCDD in peanut oil.  Peanut oil is not a hazard nor is the amount of anisole present in the solution.

Chemical signature and SARs.  In our laboratory we also use the polycyclic aromatic hydrocarbons (PAHs) dimethylbenzanthracene (DMBA) and benzo[a]pyrene (BaP).  These compounds also bind the AhR with high affinity.  

Volatility.  The vapor pressure of TCDD is 7.4x10-4 mmHg at 25°C, which indicates that this compound is not volatile with such a low vapor pressure.  In comparison the vapor pressure of water at 25°C is 23.756 mmHg.  Moreover, upon adsorption to sediment, the volatilization removal half-life is greater than 50 years.

III. ADME

Absorption.  In a lipophilic vehicle, such as corn oil, 70-80% of the initial dose is absorbed in orally-exposed rodents.  In a human volunteer, >87% of administered single oral dose was absorbed.  It should be noted that passage across intestinal wall is limited by its molecular wall and solubility. Following a dermal exposure in a dose-response experiment in rats, at 0.00015 µmol/kg 38.3% was absorbed while at 1.0 µmol/kg only 17.8% of the dose was absorbed, as most TCDD could not penetrate the stratum corneum.  TCDD absorption following transpulmonary exposure is similar to oral exposure with 95% absorption with intratracheal instillation in mice. Limited information exists regarding inhalation exposure in humans.

Disposition.  Once absorbed into the bloodstream, TCDD is primarily distributed in adipose tissue with very little in the liver. However, following intratracheal administration TCDD is preferentially sequestered in the liver (33%) over fat.  In humans, ~90% of body burden following an oral dose of TCDD in corn oil was in fatty tissue.

Metabolism/excretion.  TCDD is not readily metabolized, as the half-life is 7-12 years in humans and approximately 8 days in mice.  However, within the first 24 hours of administration in mice, about 70% of the administered dose is excreted in feces and about 1-2% of the dose is excreted in urine.  Following an intratracheal dose of 0.32µg of TCDD, rats excreted 26.3% in feces in the 3 days following exposure. 

IV. Risks

Perceived.  Dioxins are reported to be carcinogenic and cause many adverse effects so LAR staff may believe that they are at risk of developing these conditions.

Actual.  During TCDD administration, investigators wear gloves so there is no risk of dermal exposure at any time.  Moreover, C57Bl/6 mice possess a high affinity AhR while humans have a low affinity AhR so at least a 10 fold greater dose in people is needed to elicit same effects seen in mice. TCDD is not volatile in nature, and the filtertop cages protects from release in the air even if it were volatile; therefore, exposure by inhalation is virtually nil.  Additionally, only one mouse is housed per caged, and the bedding used in LAR does not promote the release of particulates.

As previously mentioned, the background body burden of TCDD in the general population is 1 ng/kg (based on serum levels and average of 22% body fat) and the adverse effects of TCDD may be observed at 31-6600 ng/kg depending on age, gender, health and nutritional stats, and genetics among other factors.  Note that this dose is significantly higher than what people could potentially be exposed to when interacting with our experimental mice and their cages.

V. Route and Doses Utilized in Shepherd Lab at UM

As previously described, TCDD is dissolved in peanut oil to make a 1.5 µg/ml solution, which is brought down to LAR in a vial.  Animals receive 10 µl/g body weight (~250 µl solution) by oral gavage for a delivered dose of 15ug TCDD/kg body weight (~0.375 µg TCDD/mouse).  This dose is ~10x less than the LD50 of TCDD, as the LD50 for a mouse is ~120 µg/kg (C57Bl/6 mice), ~ 1mg/kg for hamsters, and not known in people.  The following scenario takes into account all of the aforementioned information:

-   Following gavage, each of our mice will secrete ~0.2625 µg TCDD in the feces within 24h.

-   If someone is somehow exposed to all of the TCDD in cage, the dose would be 0.0035 µg/kg bw or 3.5 ng/kg bw (based on the average human weight of 75kg).

VI. Summary of TCDD Procedures in Laboratory Animal Resources at UM

TCDD is a persistent environmental contaminant that we use in our laboratory to study the immunomodulatory effects of aryl hydrocarbon receptor activation.  Although TCDD is classified as a carcinogen, we use a very low dose in mice that does not elicit this effect.  Moreover, TCDD is not a volatile compound even when excreted so there is no significant risk of dermal or inhalation exposure. 

All animals to be treated with TCDD will be housed in SB 028. Although not a requirement (relative to the TCDD exposure) cage changing will be done in the BSL-2 biosafety cabinet located in that room although it is not a requirement.  There is no real risk associated with the amount of TCDD that could potentially end up in the tissue or bedding, and the maximum levels that could possibly be in either tissue or bedding are below all regulatory limits so that both tissue and bedding may be disposed of in the normal waste streams.

VII. References

Reports from the following agencies were used to produce this report regarding the toxicity of TCDD:

-   Agency for Toxic Substances and Disease Registry (ATSDR)

-   Environmental Protection Agency (EPA)

-   National Institute for Occupational Safety and Health (NIOSH)

-   National Toxicology Program (NTP)