Antiviral activity of a paint film is its ability, under specified conditions, to inactivate viruses or make them lose infectivity. GB/T 21866-2025 Test method for antiviral activity and antimicrobial of paints film (published 1 Aug 2025, effective 1 Feb 2026, replacing GB/T 21866-2008) brings "antiviral" into China's national testing framework for coatings. This article walks through the test framework, the calculations and the formulation-side boundaries.
TL;DR
GB/T 21866-2025 quantifies the antiviral performance of a coating film with a film-covering method plus a cell-culture infectivity assay: two model strains — influenza A (H3N2, ATCC VR-1679) on MDCK cells and enterovirus EV71 (ATCC VR-1432) on Vero cells — are inoculated on the film, eluted after 24 h, and the residual infective titer is measured by plaque assay or TCID₅₀. Each +1 in the antiviral activity value equals a tenfold (90%) drop in virus count; R = 3.0 corresponds to about 99.9% inactivation, a common industry reference line. The standard is a test method with no pass grades — claim thresholds belong in product standards or purchase contracts, and antibacterial and antiviral performance must be certified separately.
Antiviral vs antibacterial: different targets
Viruses have no independent metabolism and must replicate in living cells; bacteria and fungi metabolise on their own. Antibacterial testing (Chapter 5 of the new standard; ISO 22196:2011) counts colonies, while antiviral testing measures infective titer — a virus particle that has been inactivated but still carries intact nucleic acid can be amplified by PCR, yet no longer represents an infection risk. Chapter 3 of the 2025 edition defines "antiviral" as inactivating viruses or removing their infectivity, and deletes the procedural terms of the 2008 edition.
Published data show why surfaces matter in the transmission chain: SARS-CoV-2 can stay infectious on solids for about 7 days (Behzadinasab et al., ACS Appl. Mater. Interfaces, 2020, DOI 10.1021/acsami.0c11425). The test reproduces exactly this chain: virus suspension on the film — contact — elution — residual infectivity.
Test framework of GB/T 21866-2025
Strains, host cells and incubation
Table 1 of the standard fixes two model strains: H3N2 (enveloped) on MDCK cells at (34±1) °C, and EV71 (non-enveloped) on Vero cells at (37±1) °C, both in EMEM medium with (5±0.5)% (by volume) CO₂. Other low-pathogenicity strains may be agreed between the parties and must be reported with their names and culture-collection numbers.
Specimens and cover film
Antiviral specimens are coated panels of aluminium, glass or stainless steel, (50±2) mm × (50±2) mm and at least 1 mm thick, at least 9 pieces; control specimens are polyethylene film of the same size, at least 12 pieces. The cover film is low-density polyethylene (0.918–0.928 g/cm³), (40±2) mm × (40±2) mm, 0.05–0.10 mm thick, either plain or perforated with 144 holes (Annex A) — the perforated film lets the eluent pass through and prevents strongly adsorptive films from "locking in" virus and producing false positives. Water-absorbing films must be judged and pre-conditioned per Annex C.
Inoculation, elution and titer
The virus stock must exceed 10⁷ PFU/mL (or TCID₅₀/mL); the inoculum is diluted to 10³–10⁴ PFU/mL, and a thawed working suspension may be kept at 2–8 °C for no more than 4 h. After contact, specimens are eluted with SCDLP medium (or another neutraliser validated in a pre-test), and the cytotoxicity of the eluate on host cells is excluded first. Residual infectivity is measured by plaque assay (6-well plates) or TCID₅₀ (96-well plates).
Calculations
Antiviral rate (%) = (1 − titer of test ÷ titer of control) × 100; antiviral activity value = lg(control titer) − lg(test titer). Clauses 4.8 and 5.9 specify antiviral and antibacterial durability tests with re-testing after ageing — formulation validation should target the aged retention rate, not the initial value.
Key figures
- Testing in a BSL-2 or higher biosafety laboratory (GB 19489; biological safety cabinet per GB 41918).
- Inoculum 10³–10⁴ PFU/mL; stock titer >10⁷ PFU/mL; working suspension ≤4 h at 2–8 °C.
- Specimens (50±2) mm × (50±2) mm, ≥1 mm thick; ≥12 control and ≥9 test pieces.
- LDPE cover film (40±2) mm × (40±2) mm × 0.05–0.10 mm; media sterilised at (121±2) °C, (103±5) kPa.
- Incubation (34±1) °C (H3N2) / (37±1) °C (EV71), (5±0.5)% CO₂, 24 h contact.
- Activity value conversion: R=1 → 90%; R=2 → 99%; R=3 → 99.9% inactivation.
- Timeline: published 1 Aug 2025, effective 1 Feb 2026; the 2008 edition is withdrawn the same day.
Three standards side by side
| Item | GB/T 21866-2025 (current) | GB/T 21866-2008 (replaced) | ISO 21702:2019 (reference) |
|---|---|---|---|
| Scope | functional paint films on building and wood surfaces | antibacterial paint films | plastics and other non-porous surfaces |
| Target | viruses + bacteria | bacteria only | viruses only |
| Strains | H3N2 ATCC VR-1679 (MDCK), EV71 ATCC VR-1432 (Vero) | S. aureus AS1.89, E. coli AS1.90 | influenza A virus, feline calicivirus |
| Contact | (34±1) °C / (37±1) °C, (5±0.5)% CO₂, 24 h | (37±1) °C, RH >90%, 24 h | (25±1) °C, 24 h |
| Reported | antiviral rate / activity value; antibacterial part likewise | antibacterial rate (Grade I ≥99%, Grade II ≥90%, plus durability threshold) | antiviral activity value (log difference) |
| Type | test method, no grading | method + effect grading | test method |
| Durability | 4.8 antiviral; 5.9 antibacterial durability tests | re-test after 30 W UV lamp (253.7 nm) exposure for 100 h | no mandatory ageing procedure |
Formulation view: what supports an antiviral claim
Metal systems (ions and oxides of silver, copper and zinc) act on envelope proteins, capsids and nucleic acid; photocatalytic TiO₂ generates reactive oxygen species under light; organic systems (quaternary ammonium, polyguanidine, halamine) attack envelopes and capsids. Three points decide whether the formulation data hold up:
1. Both enveloped and non-enveloped strains
H3N2 has a lipid envelope and shorter environmental survival; EV71 is non-enveloped and more robust. Components that only work against enveloped viruses may fail on EV71 — both strains must pass for a complete claim.
2. Retention after ageing
The silver/copper release reservoir in a film is finite; light and washing consume the active surface layer, and UV can catalyse silver discoloration (grey-yellow Ag₂O/AgCl). Re-test after durability ageing and monitor colour change — that is what turns laboratory data into engineering.
3. Claim wording
The 2025 edition removed the 2008 antibacterial grading and kept only the methods. Thresholds for antiviral rate / activity value belong in product standards, company declarations or purchase contracts (a common reference is activity value ≥3.0, i.e. ≥99.9%; on the product side, HG/T 3950-2007 suggests ≥99% with ≥95% retention after ageing). Claims need third-party reports, and an antibacterial report must not be used to support an antiviral claim.
FAQ
Q1: What are the main differences from the 2008 edition?
A new antiviral activity test (Chapter 4) plus annexes for EMEM medium (Annex B), perforated films (Annex A) and water-absorption judgement/pre-conditioning (Annex C); the terms and the antibacterial effect grading of the 2008 edition are removed; the scope now covers functional paint films on building and wood surfaces, with other uses by reference.
Q2: Can an antiviral report replace an antibacterial report?
No. The targets and method systems differ, and the same component does not necessarily inactivate bacteria and viruses alike; Chapters 4 and 5 are two parallel tests. Acceptance documents should carry both reports.
Q3: Is an activity value of 3.0 the pass line in the standard?
No. The 2025 edition only specifies test and calculation methods, without grades; ≥3.0 (about 99.9%) is a common industry reference, and the actual threshold follows the product standard or contract.
Q4: Why the two strains H3N2 and EV71?
They represent enveloped and non-enveloped viruses respectively, covering both structural classes; both are low-pathogenicity laboratory strains that can be handled at BSL-2, and reports must state the collection numbers (ATCC VR-1679 / VR-1432).
Q5: What should the laboratory and panels prepare before testing?
A BSL-2 laboratory meeting GB 19489 with a GB 41918 biological safety cabinet; panels coated per GB/T 1727 and conditioned per GB/T 9278; aluminium/glass/stainless-steel substrates (50±2) mm × (50±2) mm, ≥1 mm thick, ≥9 test pieces; water-absorbing films pre-conditioned per Annex C.
Q6: Which standard applies to export products?
ISO 21702:2019 for plastics and other non-porous surfaces, ISO 18184:2019 for textiles. Strains and contact conditions differ among the three standards, so reports are not interchangeable; follow the target market's designation.
Q7: Can PCR replace the infectivity assay?
Not for conclusions on its own. Inactivated particles still amplify in PCR, which is only a screening tool; the standard bases its judgement on infective titer by plaque assay or TCID₅₀.
Last updated: 2026-10-22
References: GB/T 21866-2025 (replacing GB/T 21866-2008); GB 19489; GB 41918; GB/T 1727; GB/T 9278; ISO 21702:2019; ISO 18184:2019; ISO 22196:2011; HG/T 3950-2007; Behzadinasab S, Chin A, Hosseini M, Poon L, Ducker W A. A Surface Coating that Rapidly Inactivates SARS-CoV-2. ACS Appl Mater Interfaces, 2020, 12(31): 34723-34727. DOI 10.1021/acsami.0c11425.
Kexin New Materials (Guangdong) Co., Ltd.