What is benzo[a]pyrene?
Benzo[a]pyrene (BaP) is a five-ring polycyclic aromatic hydrocarbon (PAH) formed by the incomplete combustion of organic material — vehicle exhaust, wood smoke, cigarette smoke, grilled and smoked foods, and coal or oil burning all produce it. It is classified as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC), and it is the most studied and most commonly used reference compound for the entire class of carcinogenic PAHs. Because PAHs almost always occur as complex mixtures rather than as a single compound, environmental scientists convert a mixture's composition into a single "benzo[a]pyrene equivalent" so that samples with different PAH profiles can be compared on one scale.
The toxic equivalency factor (TEF) method
The standard way to do this conversion is the Toxic Equivalency Factor (TEF) approach. Each PAH is assigned a TEF that expresses its carcinogenic potency relative to benzo[a]pyrene, which is fixed at TEF = 1. The benzo[a]pyrene toxic equivalency (BaP-TEQ) of a mixture is then:
BaP-TEQ = Σ (Cⁱ × TEFⁱ)
where Cⁱ is the measured concentration of PAH "i" and TEFⁱ is its equivalency factor. This calculator uses the toxic equivalency factors published by Nisbet and LaGoy (1992), which remain the most widely cited TEF set in environmental screening and regulatory guidance:
- Benzo[a]pyrene (BaP): TEF = 1 (reference compound)
- Dibenzo[a,h]anthracene: TEF = 5 (roughly 5× more potent than BaP)
- Benzo[a]anthracene: TEF = 0.1
- Benzo[b]fluoranthene: TEF = 0.1
- Benzo[k]fluoranthene: TEF = 0.1
- Indeno[1,2,3-cd]pyrene: TEF = 0.1
- Chrysene: TEF = 0.01
Other bodies (the World Health Organization, California's OEHHA, and the US EPA) have published alternative TEF sets for some of these compounds, and values can differ by up to an order of magnitude for a few PAHs — so always report which TEF scheme was used alongside any BaP-TEQ figure.
Common reference and benchmark values
- EU ambient air target value for benzo[a]pyrene (as a PAH marker in PM10): 1 ng/m³ annual average (EU Directive 2004/107/EC).
- WHO does not recommend a "safe" air threshold for BaP because it is genotoxic, but uses an excess lifetime cancer risk of 1 in 100,000 at an air concentration of about 1.2 ng/m³.
- EU maximum level for benzo[a]pyrene in smoked meat and fish products: 2.0 µg/kg; for the sum of 4 PAHs (PAH4, including BaP): 12.0 µg/kg (EU Regulation 835/2011).
- Molar mass of benzo[a]pyrene: 252.31 g/mol (chemical formula C₂₀H₁₂), useful for converting between mass-based (µg/kg, ng/m³) and molar concentrations.
Why use a TEQ instead of measuring BaP alone
Regulators historically used benzo[a]pyrene alone as a marker for total PAH contamination because it is easy to measure and well studied. However, real combustion sources emit dozens of PAHs simultaneously, and some — like dibenzo[a,h]anthracene — are more carcinogenic per unit mass than BaP itself. A BaP-TEQ that sums several PAHs weighted by potency gives a more representative estimate of the mixture's overall cancer risk than the BaP concentration alone, which is why food-safety and air-quality agencies increasingly report both the raw BaP value and a multi-PAH TEQ or PAH4/PAH8 sum.