Development of an inhalation unit risk factor for isoprene

Area of Science:

• Environmental Health
• Toxicology
• Risk Assessment

Background:

• Isoprene is a volatile organic compound found in the environment and endogenously produced.
• Assessing the carcinogenic potential of isoprene is crucial for public health.
• Existing data from animal studies provide a basis for quantitative risk assessment.

Purpose of the Study:

• To develop a unit risk factor (URF) for isoprene to quantify cancer risk.
• To establish a safe lifetime air concentration for isoprene exposure.
• To compare estimated risk levels with endogenous and environmental concentrations.

Main Methods:

• Dose-response modeling using data from three key animal studies.
• Calculation of the URF based on the 95% lower confidence limit for liver carcinoma in mice.
• Application of adjustment factors for interspecies differences and dosimetry.
• Determination of the lifetime air concentration associated with a 1 in 100,000 excess risk level.

Main Results:

• A unit risk factor (URF) of 6.2E-08 per ppb was derived for isoprene.
• The lifetime air concentration for a 1 in 100,000 excess risk level was determined to be 160 ppb.
• This risk level is significantly lower than exposure levels causing cancer in animal studies.
• Environmental exposure is expected to result in a minimal increase in blood isoprene levels compared to endogenous means.

Conclusions:

• The developed URF provides a quantitative measure for isoprene-related cancer risk.
• The established safe exposure level is substantially lower than observed effect levels in animal models.
• Environmental and endogenous isoprene levels are unlikely to pose a significant excess cancer risk at the established threshold.