PAHs and Hydrocarbons

The PAH (Polycyclic Aromatic Hydrocarbons) family includes nineteen molecules in the IsoFind catalog, organized by the number of condensed rings. This includes the sixteen US EPA priority PAHs, one methylated PAH, and two six-ring PAHs of particular concern. This page presents the family structure, the sharply contrasting regulatory thresholds between light and heavy PAHs, tabulated degradation pathways, and available CSIA data, with a note on petroleum hydrocarbons yet to be integrated into the catalog.

Eight Subfamilies by Size

PAHs are grouped by the number of condensed rings, from 2 to 6. This size-based classification reflects major behavioral trends: solubility, volatility, toxicity, and persistence evolve with the number of rings. Two special subfamilies complete the classification: methylated PAHs and 4-5 ring hybrid cases.

Subfamily	Count	Molecules
2-ring PAHs	1	Naphthalene
3-ring PAHs	5	Acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene
4-ring PAHs	4	Fluoranthene, pyrene, benzo[a]anthracene, chrysene
5-ring PAHs	4	Benzo[a]pyrene, benzo[b]fluoranthène, benzo[k]fluoranthène, dibenzo[a,h]anthracene
6-ring PAHs	2	Benzo[g,h,i]perylene, indeno[1,2,3-cd]pyrene
4-5 ring PAHs	1	Benzo[j]fluoranthene
6-ring PAHs (variant)	1	Dibenzo[a,l]pyrene (Pro access)
Methylated PAHs	1	5-Methylchrysene

Eighteen of the nineteen PAHs require Pro access. Only naphthalene remains free to access, consistent with its role as an introductory archetype for the family.

Sharply Contrasting Regulatory Thresholds

The contrast in regulatory thresholds between light and heavy PAHs is one of the most pronounced in the catalog. It ranges from 2.4 µg/L for naphthalene to 0.00082 µg/L for six-ring PAHs—a difference of nearly four orders of magnitude. This hierarchy reflects the relationship between molecular weight, carcinogenic potential, and bioaccumulation.

Threshold Category	AA-EQS Value	Affected PAHs
Mobile light PAH	2.4 µg/L	Naphthalene
Fluoranthene alone	0.12 µg/L	Fluoranthene (+ biota EQS 30 µg/kg)
Standard priority PAHs	0.10 µg/L	Anthracene
Carcinogenic 4-ring PAHs	0.05 µg/L	Benzo[a]anthracene, chrysene
Dangerous carcinogenic PAHs	0.017 µg/L	Benzo[a]pyrene, B[b]F, B[k]F, B[j]F, Dibenzo[a,h]anthracene, Dibenzo[a,l]pyrene
Heavy 6-ring PAHs	0.00082 µg/L	Benzo[g,h,i]perylene, indeno[1,2,3-cd]pyrene (+ biota EQS 0.085 µg/kg)

Benzo[a]pyrene carries two distinct thresholds depending on the matrix: 0.017 µg/L for AA-EQS (surface water, WFD 2013/39/EU) and 0.010 µg/L for drinking water (Dir. 98/83/EC). The drinking water threshold is stricter. IsoFind systematically normalizes to µg/L to allow comparison, but the user must choose the regulatory framework consistent with the analyzed matrix.

The Sixteen EPA Priority PAHs

The list of sixteen priority PAHs published by the US Environmental Protection Agency remains a worldwide reference for contamination assessments. It is fully covered by the IsoFind catalog.

Molecule	Formula	Molar Mass	IARC Status
Naphthalene	C₁₀H₈	128.17	Group 2B
Acenaphthylene	C₁₂H₈	152.19	Not classified
Acenaphthene	C₁₂H₁₀	154.21	Group 3
Fluorene	C₁₃H₁₀	166.22	Group 3
Phenanthrene	C₁₄H₁₀	178.23	Group 3
Anthracene	C₁₄H₁₀	178.23	Group 3
Fluoranthene	C₁₆H₁₀	202.25	Group 3
Pyrene	C₁₆H₁₀	202.25	Group 3
Benzo[a]anthracene	C₁₈H₁₂	228.29	Group 2A
Chrysene	C₁₈H₁₂	228.29	Group 2A
Benzo[a]pyrene	C₂₀H₁₂	252.31	Group 1 (Reference)
Benzo[b]fluoranthene	C₂₀H₁₂	252.31	Group 2B
Benzo[k]fluoranthene	C₂₀H₁₂	252.31	Group 2B
Dibenzo[a,h]anthracene	C₂₂H₁₄	278.35	Group 2A
Benzo[g,h,i]perylene	C₂₂H₁₂	276.33	Group 3
Indeno[1,2,3-cd]pyrene	C₂₂H₁₂	276.33	Group 2B

Benzo[a]pyrene serves as the carcinogenic reference for the entire group: it carries a Toxic Equivalence Factor (TEF) of 1, and other carcinogenic PAHs are expressed in BaP-equivalents with weighted TEFs. This convention allows for the calculation of a global BaP equivalent even when individual concentrations are all below strict thresholds.

Non-EPA Listed PAHs

Three PAHs in the catalog go beyond the classic EPA list. They are maintained for specific reasons.

Molecule	Role	Status
Benzo[j]fluoranthene	WFD co-threshold with B[b]F and B[k]F	IARC Group 2B; same AA-EQS 0.017 µg/L
Dibenzo[a,l]pyrene	Carcinogenic potential higher than BaP	IARC Group 2A; TEF estimated between 1 and 10; Pro access
5-Methylchrysene	Methylated PAH, marker of incomplete combustion	IARC Group 2A; EU Methylated PAH watch list; Pro access

Tabulated Degradation Pathways

Three degradation pathways are currently tabulated for PAHs, all biological and all leading to possible mineralization. These concern naphthalene and phenanthrene, the two most studied PAHs in CSIA literature. PAHs with four or more rings do not yet have tabulated pathways due to a lack of sufficient usable data.

Molecule	Pathway	Environment	Typical t½ (d)	Primary Metabolite	Mineralization
Naphthalene	Aerobic oxidation (naphthalene dioxygenase)	Aerobic Eh +100 to +400 mV	50	Salicylate	Yes
Naphthalene	Anaerobic sulfate-reducing degradation	Anaerobic Eh −200 to 0 mV	300	2-Naphthoate	Yes
Phenanthrene	Aerobic oxidation	Aerobic Eh +100 to +400 mV	100	1-Hydroxy-2-naphthoate	Yes

The contrast between aerobic and anaerobic pathways is striking for naphthalene: the half-life increases from 50 days to 300 days—a factor of 6. This variation explains why local redox geochemistry is decisive for interpreting a PAH plume. The metabolites produced are chemically distinct (aromatic salicylate vs. carboxylated 2-naphthoate), providing useful diagnostic markers when they can be measured.

Metabolite Cascades

Tabulated parent-metabolite relationships are limited to the three known pathways. Metabolites are all of intermediate stability (neither immediately degraded nor persistent), and none are classified as toxic in the database, distinguishing them from PFAS cases where all metabolites remain persistent and toxic.

Parent	Metabolite	Formula	Max Yield	Stability
Naphthalene	Salicylate	C₇H₆O₃	0.60	Intermediate
Naphthalene	2-Naphthoate	C₁₁H₈O₂	0.70	Intermediate
Phenanthrene	1-Hydroxy-2-naphthoate	C₁₁H₈O₃	0.50	Intermediate

CSIA Data for PAHs

Four isotopic fractionations are tabulated for PAHs, all regarding naphthalene and phenanthrene. The key finding is that δ²H provides fractionation ten to fifty times higher than δ¹³C: it is the preferred diagnostic signal for this family.

Molecule	Element	ε (‰)	Range	Study	Reference
Naphthalene	C (aerobic)	-0.9	[-1.5; -0.3]	Laboratory	Morasch et al., 2011
Naphthalene	C (sulfate-reducing)	-1.2	[-1.8; -0.7]	Laboratory	Safinowski et al., 2006
Naphthalene	H	-50.0	[-80; -30]	Laboratory	Morasch et al., 2011
Phenanthrene	C	-0.7	[-1.2; -0.3]	Laboratory	Morasch et al., 2011

The scientific note attached to naphthalene's δ²H fractionation in the database is explicit: "δ²H is much more diagnostic than δ¹³C for PAHs." This hierarchy stems from the number of hydrogens involved in the rate-limiting steps of degradation, and the fact that C-H bonds are directly broken during transformation. In a PAH assessment, leveraging δ²H provides significantly more information than δ¹³C alone, where analytical capabilities allow.

Phenanthrene as a Petroleum Source Marker

Phenanthrene occupies a unique position in the family. The regulation field of its datasheet describes it as a "petroleum source marker." While included in the 16 EPA priorities, it lacks a specific WFD AA-EQS, limiting its direct regulatory use. However, its ratio with anthracene (an isomer with the same C₁₄H₁₀ formula) is a classic source diagnostic.

Phenanthrene / Anthracene Ratio	Usual Interpretation
> 10	Petrogenic source (crude oil, unburned refined products)
< 10	Pyrogenic source (combustion, high temperature)

Other classic diagnostic ratios exist (fluoranthene/pyrene, benzo[a]anthracene/chrysene, indeno/benzo[g,h,i]perylene); they are detailed in the dedicated page for inorganic geochemistry diagnostic ratios, as the principle is analogous to that used for trace elements.

Petroleum Hydrocarbons: Current Catalog Scope

The page is titled "PAHs and Hydrocarbons" because it is intended to eventually cover aliphatic and monoaromatic petroleum hydrocarbons: BTEX (benzene, toluene, ethylbenzene, xylenes), TPH (Total Petroleum Hydrocarbons), and C10-C40 index hydrocarbons. In the current version of the catalog, only polycyclic PAHs are integrated. BTEX and TPH are managed manually in an individual sample's Elemental Geochemistry view, without automatic links to the CSIA engine or the Nexus bridge.

Extending the catalog to BTEX is under study and will be consistent with other families: WFD thresholds (benzene 10 µg/L drinking water, toluene 700 µg/L, etc.), documented aerobic and anaerobic degradation pathways, and rich isotopic fractionation for both δ¹³C and δ²H. The family is well-covered by CSIA literature and will directly benefit from coupling with the simulation engine.

API Access

PAH data is accessible via the molecule module endpoints by filtering on the family.

Endpoint	Usage
GET /api/molecules/reference/catalogue?famille=HAP	List of the 19 reference PAHs
POST /api/molecules/catalogue/seed/HAP	Pre-fills the project with the PAH family
GET /api/molecules/csia/Naphtalène/pathways	Lists tabulated degradation pathways for naphthalene
POST /api/molecules/csia/resolve	CSIA resolution with preference for element H or C
POST /api/molecules/csia/dual	Dual isotope δ¹³C / δ²H for mechanistic diagnostics

Learn More

CSIA Isotopy: Principles of compound-specific fractionation, useful for understanding the advantage of δ²H for PAHs.
Degradation Pathways: Full set of 50 tabulated pathways.
Diagnostic Ratios: Petrogenic vs. pyrogenic ratios and other source diagnostics.
Reference Base: Detailed structure of the molecular catalog.