Every drug product sold in the ICH regions now carries an elemental impurity dossier. The framework behind it, ICH Q3D elemental impurities, replaced the historic heavy metals limit test in the major pharmacopoeias and brought in a risk-based, compound-by-compound methodology. The transition is complete in theory, but deficiency letters around elemental impurities are still common, particularly for legacy products where the risk assessment never received the scrutiny it warranted. This guide explains how to build an ICH Q3D elemental impurities dossier that holds up in 2026.
Why ICH Q3D elemental impurities assessment matters
Heavy metals and other elemental contaminants are toxicologically meaningful at ng-to-µg daily doses. Arsenic, cadmium, lead, and mercury in particular are of broad public-health concern, and platinum-group elements can arise from catalytic synthesis. The pre-ICH Q3D heavy metals limit test was an imprecise colorimetric assay that missed the most toxicologically relevant elements. ICH Q3D replaced it with a 24-element framework based on Permitted Daily Exposure (PDE) values derived from toxicological data.
The economic impact is tangible: a rigorous ICH Q3D elemental impurities risk assessment can eliminate routine batch testing for most elements in most products, reserving analytical effort for genuinely relevant risks.
The 24-element universe of ICH Q3D
Class 1: always evaluate
Arsenic, cadmium, lead, and mercury are human toxicants with no known essential biological role. Their PDEs are set low (5, 5, 5, and 30 µg/day respectively for oral products) and they must always be included in the risk assessment.
Class 2A: route-dependent toxicants
Cobalt, vanadium, and nickel have higher PDEs but remain significant hazards especially for parenteral and inhalation routes. Their inclusion is mandatory when any intentional or likely source exists.
Class 2B: excluded unless deliberately introduced
Silver, gold, palladium, platinum, iridium, osmium, rhodium, ruthenium, selenium, and thallium are evaluated only when intentionally used, typically as catalysts. Most small-molecule oral products exclude Class 2B after documented risk assessment.
Class 3: oral route negligible, parenteral requires evaluation
Antimony, barium, chromium, copper, lithium, molybdenum, and tin have oral PDEs high enough that contamination from excipients and equipment is typically inconsequential. Parenteral products, however, must evaluate them explicitly.
The ICH Q3D elemental impurities risk assessment workflow
Step 1: Source identification
Catalogue every potential source of each relevant element: API synthesis (intentional and incidental), excipients, water, manufacturing equipment, and container closure systems. This is the most time-consuming step, regulators expect a demonstrable inventory, not a sampling exercise.
Step 2: Evaluation against PDE
For each element and source, estimate the daily contribution based on typical concentrations and the maximum daily dose. The sum of contributions across sources is compared to the PDE. If the total is below 30% of the PDE, routine testing is not required (“control threshold” rule).
Step 3: Decision on controls
Where the total exceeds the control threshold, either routine batch testing or a supply-chain control (specification on raw materials, for example) must be introduced. The ICH Q3D elemental impurities framework allows flexibility on where the control sits.
Step 4: Analytical method development
ICP-MS is the industry standard for elemental impurities analysis. Methods must be validated per USP <233> or equivalent, and the detection limit must be below 30% of the control threshold concentration.
Step 5: Documentation and review
The risk assessment document is a living artefact. Any change in synthesis route, excipient supplier, or dosing regimen may invalidate prior conclusions. Annual review is best practice even where no change has occurred.
Deriving Permitted Daily Exposure values
PDEs in ICH Q3D were derived from modern toxicological databases using NOAEL-based uncertainty factor approaches (F1 species, F2 individual variability, F3 subchronic-chronic, F4 severity, F5 toxicodynamic confidence). The published PDEs cover oral, parenteral, and inhalation routes. For any element not listed in ICH Q3D, increasingly relevant as novel catalysts enter the industry, a compound-specific PDE must be derived de novo, following the same uncertainty factor framework. The EMA ICH Q3D implementation page is the authoritative reference for current PDE values.
Where sparse toxicological data complicate derivation, read-across from structurally related elements is an option. Our Read-Across Assessment workstream builds these derivations with the documentation regulators expect to see.
Special cases in ICH Q3D elemental impurities assessment
Parenteral products
PDEs for parenteral administration are typically 10× lower than oral for the same element. The control threshold therefore sits 10× lower as well, which often forces routine testing for elements that are trivially dismissed in oral products.
Inhalation products
Inhalation PDEs depend on particle fate and local lung dose. ICH Q3D provides initial values but compound-specific refinement is common, particularly for drug-device combination products.
Large-molecule biologics
Biologics are typically low-dose and highly purified, but cell culture media, purification resins, and container systems can introduce elemental impurities. ICH Q3D applies, the risk assessment must address each source with the same rigour as small-molecule products.
Herbal products and complex mixtures
Natural-source APIs often carry background levels of trace elements from the source material. The ICH Q3D elemental impurities framework still applies, and the risk assessment must reconcile natural variability with the PDE.
Common pitfalls
Pitfall 1: Ignoring container closure contributions. Glass, rubber stoppers, and plastic bottles can shed trace elements over shelf life. A risk assessment that omits CCS inputs is incomplete.
Pitfall 2: Using single-batch analytical data as the risk basis. Regulators expect multi-batch evidence where sources are variable, especially for mined excipients.
Pitfall 3: Misapplying the control threshold. The 30% rule is a default, it can be raised to 100% of PDE if a valid supply-chain control is in place. Conversely, it cannot be raised for Class 1 elements without a compound-specific justification.
Pitfall 4: Failing to refresh the assessment after change. Excipient supplier change, API route change, or site transfer all require reassessment.
Pitfall 5: Treating ICH Q3D as a one-time filing. It is a lifecycle obligation, not a submission document.
How ToxLibrary supports ICH Q3D programmes
A defensible ICH Q3D elemental impurities assessment depends on trustworthy PDE derivations, accurate source inventories, and analytical methods that match the control strategy. Our Toxicology Monographs cover elemental impurities across all administration routes with derivation histories and regulatory citations. Where a non-ICH element requires a de novo PDE, our toxicologists deliver derivations with the EMA-style uncertainty factor documentation. Reach out, elemental impurity dossiers that anticipate reviewer questions avoid the most expensive kind of rework.
