Veterinary Taxicology

CHRONICITY FACTOR

• The ratio of acute to chronic LD 50 doses is known as chronicity factor.
• Compounds with cumulative effects have a high chronicity factor.
• A chronicity factor greater than 2.0 indicates a relatively cumulative toxicant.
• Chronicity factor may be influenced by the tendency to accumulate vs. being rapidly eliminated or detoxified .
• It may also be influenced by cumulative and progressive damage that occurs from repeated toxic insults to a target tissue.
• A compound may have low acute toxicity, but if it has the tendency to accumulate in body tissues it can cause sub acute or chronic toxicity. Such toxicants are termed as cumulative poisons.
• The chronicity factor gives an indication of cumulative effects of poisons.

• However, chronicity factor may be influenced by the tendency to produce tolerance.
• The biological system may tolerate higher dose after prolonged exposure. Potassium cyanide is an example. Its acute LD₅₀ is 10 mg/kg, but pre-exposed animals tolerate 250 mg/kg. Therefore the chronicity factor is 0.04 for potassium cyanide.