What Recent Research Tells Us About Indoor Air Management

An analysis of emerging evidence on HEPA filtration in educational settings and current UK policy approaches

Executive Summary

Recent school closures due to illness outbreaks highlight ongoing challenges in managing indoor air quality in educational settings. This article examines peer-reviewed research on air filtration interventions, current government provision levels, and successful local authority implementations to understand what works and what gaps remain in national policy.

The Current Situation

Recent weeks have seen significant illness-related disruptions across UK schools:

• St Martin’s Comprehensive School in Caerphilly closed from December 5-9, 2025, following 242 pupils and 12 staff reporting flu-like illness
• Ebrington Primary School in County Londonderry reported 170 pupils absent on December 8, 2025
• DfE statistics show persistent absence rates of 17.79% (autumn 2024/25), remaining above pre-pandemic levels. Illness continues to account for the majority of school absences

These incidents raise important questions about whether cost-effective interventions exist to reduce transmission in educational settings.

The Bradford Class-ACT Study: What We Learned

Between September 2021 and April 2022, researchers from the University of Leeds conducted a randomized control trial across 31 Bradford primary schools to evaluate HEPA air filtration effectiveness.

Study Design

• Conducted across 31 Bradford primary schools (10 HEPA, 10 UV, 11 controls)
• Funded by the Department of Health and Social Care
• Managed through UK Health Security Agency
• DfE officials participated in the working group
• Conducted between September 2021 and April 2022 during Delta and Omicron waves

Key Findings Reported by Principal Investigator Professor Catherine Noakes

• Schools with HEPA filters experienced over 20% fewer illness-related absences compared to control schools
• Results presented at WHO Europe Indoor Air Conference (September 2023)
• Evidence submitted to UK COVID-19 Inquiry

Important context: The full peer-reviewed paper is still in process, so these findings should be considered preliminary but promising. The research was conducted with government participation and oversight.

Current Government Provision: An Analysis

Following the Class-ACT study and related research, the DfE implemented a limited HEPA distribution program:

What Was Provided

• 8,026 air cleaning units delivered by June 2022
• 386,699 CO₂ monitors distributed to schools
• Units targeted at specific poorly-ventilated spaces

Scale Analysis

• 24,479 schools in England (DfE 2024/25 data)
• Distribution works out to approximately 0.3 units per school
• DfE guidance explicitly states: “will not pay for replacement filters”

Policy Framework

• Government guidance emphasizes that “air cleaning units are not a substitute for ventilation”
• Units positioned as supplementary measure, not primary solution
• Ongoing costs shifted to individual school budgets

The Cost-Benefit Question

Industry analysis and parliamentary costings provide useful benchmarks for understanding scale:

2021 Parliamentary Petition Costings

• Approximately £500 per classroom for HEPA units
• Scaled across ~300,000 classrooms in England: ~£140 million total
• Or roughly £5,700 per school for comprehensive coverage

Context

• UK education spending: £116 billion annually (IFS 2024/25)
• £140 million represents 0.12% of total education budget
• For comparison: NHS Test and Trace budget was £37 billion

What Research Suggests This Investment Could Deliver

• 20%+ reduction in illness-related absences (Bradford study findings)
• 99.97% filtration efficiency for viral particles (HEPA standard)
• Reduced allergen and pollutant exposure
• Potential cognitive and learning benefits (2021 Lancet Commission review)

Case Study: Hertfordshire’s Comprehensive Approach

While national rollout has been limited, some local authorities have implemented broader programs:

Hertfordshire County Council (2024)

• HEPA filters installed in every classroom across 63 primary schools
• Partnership with SAMHE air quality monitoring project
• Estimated investment: £5,000-£7,000 per school

Early Evaluation Results

• Approximately 30% reduction in classroom particulate matter
• Greater improvements observed on high outdoor pollution days
• Full illness-absence analysis for 2024/25 academic year ongoing

This real-world deployment provides valuable data on implementation feasibility and early indicators of effectiveness.

London’s City-Wide Rollout

While Hertfordshire demonstrated local authority implementation, London has recently launched the UK’s largest regional air quality programme:

Mayor of London’s School Filters Programme (2025)

• £2.7 million investment announced October/November 2025
• 200 schools across London receiving HEPA filtration systems
• Programme specifically targets reduction of PM2.5 pollution and respiratory illness
• Represents first city-wide deployment in the UK
• Implementation ongoing through 2025/26 academic year

This development significantly shifts the UK implementation landscape, demonstrating growing regional appetite for air quality interventions even as national policy remains cautious.

Current UK Implementation Summary (December 2025)

• National (DfE): Limited deployment (8,026 units distributed 2021-22); no universal programme
• London (Regional): Active rollout to 200 schools (launched late 2025)
• Hertfordshire (Local): Implemented across 63 primary schools (2024-25)
• Bradford (Research): Trial completed; influencing policy but no city-wide rollout confirmed

Understanding the Infrastructure Challenge

Several factors contribute to transmission risk in educational settings:

Building Infrastructure

• Many UK schools operate in Victorian-era buildings
• Original ventilation systems designed for different occupancy standards
• Winter ventilation challenges (balancing fresh air with thermal comfort)

Occupancy Patterns

• State school average infant class size: 26.2 pupils
• Primary classes overall in mid-20s
• Private school pupil-teacher ratios: 8-12
• Higher occupancy correlates with greater transmission risk

Budget Constraints

• Schools facing multiple cost pressures (energy, staffing, maintenance)
• Limited capital budgets for infrastructure improvements
• Difficult trade-offs between competing priorities

The Science of Indoor Air Quality

HEPA filtration technology has a well-established evidence base:

Technical Foundations

• Developed in 1940s for Manhattan Project (radioactive particle containment)
• CDC/EPA standard: ≥99.97% efficiency at 0.3 micrometers
• Effective for human-generated viral aerosols

Evidence Base: Current State of Research

The research picture on HEPA effectiveness in schools is promising but still developing:

Supporting Evidence:

• Bradford Class-ACT Study: Preliminary findings show 20%+ reduction in illness-related absences in HEPA-equipped schools
• Hertfordshire SAMHE Project: Robust monitoring demonstrates 30% reduction in classroom particulate matter; illness-absence data collection ongoing
• Swiss Open Forum Infectious Diseases Study (2024): HEPA cleaners reduced particle concentrations by 77% and showed signals of reduced respiratory infection absences
• Cambridge University Hospitals: HEPA filters reduced airborne SARS-CoV-2 to undetectable levels in COVID ward

Emerging Contrary Evidence:

• JAMA Network Open 2025 Study (Sun et al.): Large cluster-randomised trial across 200 US classrooms in 39 schools found HEPA purifiers were not associated with overall reduction in high respiratory viral exposure, though they did reduce viral diversity. Authors suggest multicomponent approaches beyond filtration alone may be needed.

Current Scientific Consensus:

• Strong evidence that HEPA units significantly reduce airborne particle concentrations in classrooms
• Promising but not yet conclusive evidence for reduced illness transmission across all school settings
• Effectiveness may vary based on factors including ventilation rates, occupancy, device placement, and maintenance
• Most benefit likely when integrated into comprehensive air quality strategies rather than as standalone intervention

Important clarification: Professor Noakes notes: “Air cleaning technologies are not the silver bullet to controlling infection: they can add to ventilation, but they can’t replace it.”

This positions HEPA as one tool in a comprehensive air quality strategy, not a standalone solution.

Policy Gap Analysis

Current situation presents several areas for consideration:

What We Know

• Research demonstrates 20%+ reduction in illness absence (preliminary findings)
• Technology is proven and commercially available
• Local authority implementations show feasibility
• Cost estimates suggest 0.12% of education budget for comprehensive rollout

What Remains Unclear

• Why limited national deployment following positive trial results
• How ongoing filter replacement costs would be managed at scale
• Whether cost-benefit analysis has been conducted at national level
• What threshold of evidence would trigger broader implementation

Economic Considerations

• Illness-related school closures create significant economic costs (parental work disruption, emergency childcare)
• 17.79% persistent absence rate represents substantial lost learning time
• Potential productivity benefits of reduced illness may offset capital costs

Looking Forward: Questions for Policymakers

Rather than assign blame, it’s worth considering key questions that could inform future policy:

1. Evidence review: Given Bradford study findings and Hertfordshire early results, would a comprehensive evidence review be appropriate?
2. Pilot expansion: Could successful local authority programs be expanded as proof-of-concept before national rollout?
3. Cost-benefit analysis: Has detailed economic modeling been conducted comparing capital costs against savings from reduced absence?
4. Replacement filter funding: What sustainable funding model could address ongoing maintenance costs?
5. Implementation standards: What technical standards and deployment guidelines would ensure effectiveness?
6. Integration with ventilation: How can air filtration complement (not replace) improved mechanical and natural ventilation?

Practical Considerations for Schools

For individual schools considering air quality interventions:

Assessment Steps

1. Evaluate current ventilation adequacy using CO₂ monitoring
2. Identify poorly-ventilated spaces that can’t be improved structurally
3. Consider HEPA as supplementary to (not replacement for) ventilation
4. Factor in ongoing filter replacement costs
5. Look for case studies from similar settings (e.g., Hertfordshire)

Funding Approaches

• Explore local authority programs
• Consider energy efficiency grants (improved ventilation may reduce heating costs)
• Parent-Teacher Associations and community fundraising
• Multi-academy trust level procurement for better pricing

Industry Perspective

As a respiratory protection equipment supplier, we observe these developments with professional interest. Our experience across healthcare, industrial, and now educational sectors suggests several insights:

Implementation matters: Technology effectiveness depends heavily on proper selection, placement, and maintenance. The difference between a successful deployment and wasted investment often lies in these details.

Sustainability is key: One-time capital investments without ongoing support typically fail. Hertfordshire’s success likely reflects commitment to proper maintenance and filter replacement.

Integration, not isolation: Air quality interventions work best as part of comprehensive infection control strategies, not as standalone solutions.

Evidence drives adoption: Clear, peer-reviewed evidence will be essential for broader acceptance and funding justification.

Conclusion: From Evidence to Action

The evidence base for HEPA filtration in schools is developing and generally promising. The Bradford study, Hertfordshire deployment, and supporting research suggest these systems can reduce airborne particles and may lower illness-related absences, though results vary across studies and settings. The recent London programme demonstrates growing regional momentum for air quality interventions.

The question is: how do we move from successful pilots and regional programmes to evidence-based national policy?

This requires:

• Clear evidence synthesis and publication
• Honest cost-benefit analysis
• Sustainable funding models
• Technical implementation standards
• Political will to prioritize indoor air quality

As we’ve seen with water quality, sewage treatment, and fire safety, establishing baseline standards for indoor air quality in schools is achievable, if we decide it’s a priority.

The recent school closures remind us that this isn’t just about preventing future pandemics. It’s about reducing the everyday disruption that seasonal illness causes to children’s education and families’ lives.

Sources and Methodology

Primary Research

• Bradford Class-ACT Study (Prof. Catherine Noakes, University of Leeds)
• Findings reported at WHO Europe Indoor Air Conference (September 2023)
• Written evidence to UK COVID-19 Inquiry
• Hertfordshire SAMHE air quality monitoring project (samhe.org.uk / UKHSA)

Government Data

• DfE School Census Data 2024/25
• DfE statistics on delivery of air cleaning units (8,026 delivered by June 2022)
• DfE Pupil Absence Statistics (Autumn 2024/25)
• GOV.UK: “Using CO₂ monitors and air cleaning units in education settings”

Cost Analysis

• 2021 Parliamentary petition and Liberal Democrat costings for classroom air purifiers
• Institute for Fiscal Studies: UK education spending analysis 2024/25
• Industry pricing data for HEPA units and replacement filters

Supporting Research

• Cambridge University Hospitals HEPA filtration study
• Lancet COVID-19 Commission Task Force: “Designing infectious disease resilience into school buildings”
• CDC/EPA HEPA filtration standards

Current Outbreak Reporting

• The Sun: St Martin’s Caerphilly (December 2025)
• BBC: Ebrington Primary (December 2025)
• Metro: Ebrington Primary (December 2025)

London Air Quality Programme

• Mayor of London: School Filters Programme (October/November 2025)
• Greater London Authority: £2.7m investment announcement

Additional Research

• JAMA Network Open: Sun et al. (2025) – Cluster-randomised trial of HEPA purifiers in US schools

Note on figures: The Bradford study’s 20%+ reduction figure comes from conference presentations and inquiry evidence; full peer-reviewed publication is pending. Cost estimates are based on 2021 parliamentary costings and industry analysis, representing order-of-magnitude estimates rather than precise quotes. Hertfordshire per-school costs are estimated based on typical deployment scenarios.

About This Analysis

This article focuses on government policy regarding state schools in England. Healthcare, social care, and devolved administrations operate under different frameworks and funding mechanisms. The analysis reflects our professional perspective as respiratory protection specialists observing developments in air quality management across multiple sectors.

Complete Source Links

Bradford Class-ACT Study

• https://www.isrctn.com/ISRCTN10606609 (Trial registration)
• COVID-19 Inquiry Evidence (Prof. Noakes witness statement)
• WHO Europe Indoor Air Conference (Sept 2023 – Noakes presentation at ~2h50min mark)

Hertfordshire SAMHE Project

• https://www.sei.org/projects/samhe_hsvp/ (SAMHE+HSVP project overview)
• https://www.sei.org/projects/samhe/ (Main SAMHE project page)

Cambridge Hospital Study

• Journal of Hospital Infection (HEPA filtration in COVID ward)

Lancet Commission

• Lancet Public Health (COVID-19 Commission Task Force on Safe Work, Safe School, and Safe Travel)

DfE School Data

• School Pupils and Characteristics 2024/25
• Delivery of Air Cleaning Units
• Pupil Absence Statistics Autumn 2024

Government Guidance

• Using CO₂ monitors and air cleaning units (DfE guidance)

Education Budget

• IFS Green Budget October 2024 (Education spending analysis)

Test & Trace Spending

• National Audit Office Test and Trace Update

Parliamentary Petition (2021)

• Petition for air purifiers in schools (with costings)

Recent School Outbreak Reporting

• The Sun: St Martin’s Caerphilly (Dec 2025)
• BBC: Ebrington Primary (Dec 2025)
• Metro: Ebrington Primary (Dec 2025)

London Programme

• Mayor of London: School Filters Programme (October/November 2025)

JAMA Study

• JAMA Network Open (2025 HEPA purifier trial – Sun et al.)

Technical Standards

• EPA: What is a HEPA Filter?
• CDC Ventilation Guidance

Additional Supporting Sources

• NASUWT Union Guidance
• National Education Union
• Corsi-Rosenthal Foundation: Class-ACT analysis