Solvent Residue Limits: Understanding Class 1, 2, and 3 Solvents in Flavor Concentrates for Vaping

Author: R&D Team, CUIGUAI Flavoring
Published by: Guangdong Unique Flavor Co., Ltd.
Last Updated: Sep 26, 2026
WhatsApp & Telegram: +86 189 2926 7983
Email: info@cuiguai.com

Solvent Residue Limits in Vape Flavor Concentrates: ICH Q3C Class 1, 2 and 3 Solvents Explained

Residual solvents are organic volatile chemicals that are used or produced during the manufacture of an ingredient and are not completely removed by practical processing techniques. In flavor production, extraction solvents such as ethanol, hexane, and acetone can remain at trace levels in concentrates; in vaping, those residues are heated, aerosolized, and inhaled. The internationally harmonized framework for controlling them is ICH Q3C(R8), which classifies solvents into three risk tiers — Class 1 (to be avoided), Class 2 (to be limited), and Class 3 (low toxic potential) — and expresses exposure control through permitted daily exposure (PDE) values.

This article explains the ICH Q3C classification, the concentration limits for each class, how the PDE concept translates into parts-per-million limits, the analytical methods that detect these residues, and how e-liquid brand owners and importers should apply the framework to flavor concentrate specifications.

Why Residual Solvent Control Belongs in Flavor Quality Programs

Flavor concentrates sit at the top of the e-liquid supply chain, and they are the ingredients most likely to carry solvent residues. Natural extracts are frequently produced by solvent extraction; citrus oils, vanillas, tobaccos, and botanical essences are all obtained using carriers that must later be removed. When removal is incomplete, the concentrate enters the finished liquid carrying a trace of the process solvent.

The route of exposure changes the stakes. ICH Q3C states the governing principle bluntly: since there is no therapeutic benefit from residual solvents, all residual solvents should be removed to the extent possible to meet product specifications, good manufacturing practices, or other quality-based requirements. For an inhaled product, that principle is reinforced by the direct delivery of any remaining volatile solvent into the airways with the aerosol.

Solvent residues also interact with sensory quality and device performance. High-boiling or reactive residues can degrade coil life, contribute off-notes, or shift over storage time. Residual solvent control is therefore a quality parameter, not only a toxicology exercise.

ICH Q3C(R8): The Global Reference Framework

ICH Q3C, Impurities: Guideline for Residual Solvents, is a harmonized guideline of the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use, first adopted in 1997 and currently maintained at Revision 8, dated 22 April 2021. Its objective is to recommend acceptable amounts of residual solvents for the safety of the patient and to recommend the use of less toxic solvents.

The guideline defines a permitted daily exposure (PDE) as the maximum acceptable intake per day of a residual solvent in a product, expressed in mg/day. Each solvent is assigned to one of three classes based on risk assessment:

Class 1 solvents: solvents to be avoided — known human carcinogens, strongly suspected human carcinogens, and environmental hazards.

Class 2 solvents: solvents to be limited — non-genotoxic animal carcinogens, possible causative agents of other irreversible toxicity such as neurotoxicity or teratogenicity, and solvents suspected of other significant but reversible toxicities.

Class 3 solvents: solvents with low toxic potential — no health-based exposure limit is needed, and their PDEs are 50 mg or more per day.

The guideline also offers two options for setting Class 2 limits. Option 1 uses the fixed concentration limits in ppm published in the guideline’s Table 2, calculated with the equation Concentration (ppm) = 1000 × PDE / dose, assuming a product mass of 10 g administered daily. Option 2 allows the PDE to be combined with the known maximum daily dose when that dose is fixed. For flavor concentrates, Option 1 limits are the practical reference because the exact daily dose of a finished e-liquid is not a controlled variable.

Class 1 Solvents: To Be Avoided

Regulatory Rationale for the Zero-Tolerance Approach

Class 1 solvents should not be employed in manufacture at all, and where their use is unavoidable their levels are restricted to single-digit parts per million. The limits exist because these solvents are known or strongly suspected human carcinogens, with 1,1,1-trichloroethane listed on environmental grounds. Benzene at 2 ppm is the most exacting limit in the table and is the reason that any raw material produced with hydrocarbon-derived carriers requires a dedicated, sensitive analytical method: a generic purity certificate that does not quantify benzene to 2 ppm provides no assurance.

For the flavor industry, Class 1 solvents are essentially a zero-tolerance category. They should never appear in a concentrate specification, and a certificate of analysis that reports them as not detected should state the detection limit used. Not detected at 2 ppm is meaningful; not detected at 100 ppm is not.

Class 2 Solvents: To Be Limited

Class 2 solvents are associated with inherent toxicity and are limited rather than prohibited. Each has a PDE in mg/day and a corresponding Option 1 concentration limit in ppm, assuming a 10 g daily product mass. The table below reproduces selected entries from ICH Q3C(R8) Table 2.

Practical Relevance of Class 2 Solvents to Flavor Production

Several Class 2 solvents have real history in flavor production. Hexane is a classic solvent for botanical and oilseed extraction, and its 290 ppm limit reflects both its toxicity and its industrial prevalence. Methanol appears in natural extracts and denatured alcohol carriers; at 3000 ppm it is the least restricted Class 2 entry but still demands routine quantitation. Dichloromethane was once common in botanical extraction and decaffeination processes, and its 600 ppm limit still shows up in legacy flavor supply chains.

The full ICH Table 2 contains more entries than reproduced here, and Revision 8 adds PDEs for cyclopentyl methyl ether, 2-methyltetrahydrofuran, and tertiary-butyl alcohol. Specification writers should reference the current text rather than a remembered list, because PDE values have been revised over time — for example tetrahydrofuran in 2002, N-methylpyrrolidone in 2002, cumene in 2011, and methylisobutyl ketone in 2016.

Tobacco-flavored profiles deserve particular attention in this class, because natural tobacco extracts are produced by solvent processes and may carry traces of the extraction medium; our tobacco flavor concentrates are manufactured with solvent control data available on request.

ICH Q3C Class 1 and Class 2 Residual Solvent Limits: PDE and ppm Tables for Flavor Concentrates

Class 3 Solvents: Low Toxic Potential

Class 3 solvents may be regarded as less toxic and of lower risk to human health; the class includes no solvent known as a human health hazard at levels normally accepted in products. ICH states that amounts of these residual solvents of 50 mg per day or less — corresponding to 5000 ppm or 0.5% under Option 1 — are acceptable without justification. Higher amounts may also be acceptable if they are realistic in relation to manufacturing capability and good manufacturing practice.

The class is large and includes the workhorses of flavor extraction and carrier systems: ethanol, acetone, ethyl acetate, isopropyl alcohol (2-propanol), 1-propanol, methyl ethyl ketone, dimethyl sulfoxide, acetic acid, formic acid, ethyl ether, ethyl formate, methyl acetate, propyl acetate, isobutyl acetate, isopropyl acetate, heptane, pentane, the butanols, anisole, tert-butylmethyl ether, 2-methyltetrahydrofuran, and triethylamine.

Ethanol as the Dominant Extraction and Carrier Solvent

Ethanol deserves special mention. It is the dominant extraction and carrier solvent in the flavor industry because it is food-grade, volatile, and effective, and it sits squarely in Class 3. A concentrate that uses only Class 3 solvents can be qualified with a simple non-specific check: ICH explicitly allows loss on drying when only Class 3 solvents are present, which is why a supplier statement that only Class 3 solvents are likely to be present, with loss on drying less than 0.5%, is a recognized and defensible declaration.

This matters for product developers working on sweet profiles. Modern candy flavor formulation has shifted away from heavy solvents toward ethanol-based and solvent-free bases, and our article on nostalgia in candy flavor formulation discusses how that shift affects the sensory delivery of confectionery notes in vapor.

Analytical Methods: Headspace GC-MS and the 2 ppm Challenge

Residual solvents are typically determined using chromatographic techniques such as gas chromatography, as ICH states directly. In practice, static headspace gas chromatography with mass spectrometric detection (HS-GC-MS) is the standard platform for volatile solvent residues in flavor concentrates, because it separates the volatile fraction from the non-volatile matrix and delivers the sensitivity and selectivity needed at the 2 ppm benzene level.

Validation Requirements: Linearity, LOD, and LOQ

Method validation is not optional. A defensible residual-solvent method establishes linearity, accuracy, recovery, precision, and limits of detection and quantitation for each target solvent in the specific matrix. Flavor concentrates are complex, and matrix effects can suppress or inflate signals, so calibration should use authentic standards and results should be reported with the concentration basis (mg/kg or ppm), the limit of detection or quantitation, and the batch identity.

If a manufacturer or importer wants to verify a certificate, the questions to ask are: which solvents were targeted and why; what were the limit of detection and limit of quantitation; how was recovery tested; was the matrix the actual concentrate; and was the method validated for that purpose. A full residual solvent screen that does not state detection limits for benzene cannot support a Class 1 claim.

Residual Solvent Testing Methods: Headspace GC-MS Validation, LOD and LOQ for Flavor Concentrates

Applying the Framework to E-Liquid and Flavor Concentrates

ICH Q3C formally applies to pharmaceuticals, but its classification is the most widely referenced risk benchmark when flavor and e-liquid manufacturers set internal residual-solvent specifications, and the United States Pharmacopeia implements the same three-tier scheme in its General Chapter <467> Residual Solvents. For e-liquid ingredients, the framework is applied in a deliberate sequence: identify the solvents actually used or produced in each process step — ICH only requires testing for those — set limits with the Option 1 ppm values, and verify with validated methods.

Dilution Arithmetic From Concentrate to Finished E-Liquid

Dilution arithmetic matters when limits travel down the supply chain. If a concentrate is blended at 10% into a finished e-liquid, the residual-solvent concentration in the finished liquid equals 10% of the concentrate’s concentration. A concentrate that must keep acetonitrile below 410 ppm in the finished liquid can therefore contain up to 4100 ppm, while a concentrate used at a higher dosage must hold a proportionally tighter limit. Importers should specify the maximum use rate so that the concentrate limit and the finished-liquid limit are calculated on the same basis.

Different destination markets look at this differently. In the United States, a PMTA submission must characterize the product and its manufacturing controls, and our PMTA flavor master files explain how ingredient impurity data supports US applicants. China’s GB 41700-2022 governs the additive white list for domestic e-liquid, and the EU Tobacco Products Directive requires ingredient safety assessment under national implementation. In all three, a documented residual-solvent control story strengthens the file.

Across the portfolio, the same principle applies to every concentrate family — including acidic and sour flavor systems, where organic-acid carriers and their impurities deserve the same specification discipline as botanical extracts.

Risk Management and Supplier Documentation

A practical supplier statement has three tiers, mirroring ICH’s own examples in its reporting section. The first tier declares that only Class 3 solvents are likely to be present and that loss on drying is less than 0.5%. The second names the Class 2 solvents likely to be present and confirms each is below the Option 1 limit. The third, for materials where Class 1 solvents are a possibility, identifies and quantifies them at a stated detection limit.

The Three-Tier Documentation Standard

Certificates should be batch-specific, state the method and its limits of detection and quantitation, and be supported by change control. A change in extraction solvent, raw material source, or process step can alter the residue profile of a concentrate even when sensory properties are identical, so suppliers should commit in writing to notify customers of any change that affects solvent residues.

Frequently Asked Questions

What are Class 1, 2, and 3 solvents?

ICH Q3C classifies residual solvents by risk. Class 1 solvents are known or strongly suspected human carcinogens and environmental hazards that should be avoided; Class 2 solvents have inherent toxicity and are limited by permitted daily exposure; Class 3 solvents have low toxic potential and are acceptable at 50 mg per day or less, corresponding to 0.5%.

What is the difference between a PDE and a ppm limit?

PDE is the maximum acceptable intake of a residual solvent per day, expressed in mg/day. The ppm limit in the guideline’s Option 1 is the concentration that keeps daily intake at or below the PDE, assuming a 10 g daily product mass; it is calculated as 1000 times the PDE divided by the dose in grams.

Why is benzene limited to 2 ppm?

Benzene is a known human carcinogen, so ICH sets its Class 1 limit at 2 ppm, the lowest concentration limit in the guideline. That is also why analytical methods for Class 1 verification must demonstrate detection capability at 2 ppm.

Is ethanol allowed in e-liquid flavor concentrates?

Ethanol is a Class 3 solvent and is acceptable at levels corresponding to 5000 ppm (0.5%) or less without justification. It is the standard extraction and carrier solvent for food and flavor materials, and higher levels may be acceptable where justified by manufacturing capability and good manufacturing practice.

How do I know which solvents to test?

Test the solvents that are used or produced in the manufacturing or purification process, per ICH’s scope, plus any suspect legacy or cross-contamination candidates. A supplier declaration naming the solvents likely to be present is the fastest way to scope the analytical work.

Residual Solvent Certificates and Supplier Statements: What Importers Should Request for Vape Flavors

Get Technical Support and Free Samples

CUIGUAI Flavor concentrates are produced with controlled solvent profiles and can be released with residual-solvent test data, including headspace GC-MS results, on request. Our technical team will help you define concentration limits for your target market and review certificates of analysis.

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References

ICH Harmonised Guideline, Impurities: Guideline for Residual Solvents, Q3C(R8), Step 4 version dated 22 April 2021. International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use. Available at: https://database.ich.org/sites/default/files/ICH_Q3C-R8_Guideline_Step4_2021_0422_1.pdf

U.S. Pharmacopeia–National Formulary (USP–NF), General Chapter <467> Residual Solvents. USP–NF, current edition.

ICH, Q3C/Q3D Maintenance Procedure — Permitted Daily Exposure Updates. International Council for Harmonisation. Available at: https://www.ich.org/page/maintenance-procedure