For decades, developers and operators in the UK relied on the H4 Odour Management guidance — formally H4 Odour Management: How to Comply with Your Environmental Permit — to assess odour risk, prepare odour impact assessments and demonstrate compliance with odour permit conditions under the Environmental Permitting regime. However, as of 3 December 2025 the Environment Agency has withdrawn the H4 guidance and replaced it with a more modern, principle-based approach published on GOV.UK titled “Odour management: comply with your environmental permit.” The change marks a significant shift in how odour is regulated, assessed and enforced in the UK environmental permitting context.

Understanding this transition from the older H4 approach to the new guidance is essential for those preparing odour risk assessments, Odour Management Plans (OMPs), environmental permit applications, variations and compliance reports. It also affects how odour impacts are evaluated, how appropriate measures are identified, and how operators demonstrate that odour emissions from their activities do not cause pollution that adversely affects local communities.

The original H4 odour guidance was first published in the early 2000s and updated in 2009. It provided technical prescriptions for odour risk assessment, including structured assessment procedures, example odour limits and recommended tools such as dispersion modelling and sniff testing. Over time, environmental regulation and scientific understanding of odour measurement evolved, and the Environment Agency concluded that the H4 format — a relatively long PDF document with numerical benchmarks and detailed technical protocols — was out of date. 

The new guidance abandons many of the prescriptive elements that characterised H4, including fixed numeric odour concentration limits and routine sniff testing protocols, and instead focuses on a risk-based, principles-led framework. This means the emphasis is now on whether an operator is using appropriate measures and Best Available Techniques (BAT) to prevent or, where prevention is not practicable, to minimise odour pollution beyond the site boundary. This shift makes the regulatory approach more flexible but also places responsibility on operators to justify their odour management strategies through robust risk assessment, clear documentation and effective odour control measures. The new guidance is shorter, more accessible and intended to be used alongside other Environment Agency permitting guidance documents.

The modern odour management guidance is structured around a few key themes that define how odour risk should be assessed and managed for environmental permit compliance purposes. It is not simply a technical manual — it sets expectations for planning, assessment, control, documentation and ongoing management. At the heart of the new framework is the odour permit condition. Under most environmental permits, operators are required to ensure that emissions from the permitted activity are free from odour likely to cause pollution outside the site boundary unless they have used appropriate measures to prevent or, where that is not practicable, minimise odour pollution.

The guidance explains that an Environment Agency authorised officer will assess odour pollution based on what they perceive outside the boundary of the site and whether they attribute that odour to activities on site. This reflects a practical, real-world definition of odour pollution rather than reliance on numeric thresholds.

When documenting compliance, operators must be able to show that they have taken appropriate and proportionate actions to control odour emissions. These actions are typically described and justified in an Odour Management Plan (OMP) submitted with permit applications or variations and implemented as part of ongoing site management.

The new guidance re-emphasises the importance of odour risk assessments — structured evaluations of odour sources, potential exposure pathways and the likelihood that odour emissions could cause pollution. These assessments must consider the impacts and causes of past odour issues, identify and prioritise odour sources, and anticipate risks so that appropriate preventative measures can be applied. Unlike H4, the current guidance does not prescribe specific odour concentration limits or percentile thresholds. Instead, it places focus on evidence-based assessment, drawing on monitoring data where appropriate, process understanding and real-world reporting. This aligns odour assessment with broader environmental risk assessments required across other environmental media.

Odour risk assessments submitted to the Environment Agency must also be realistic and based on sound evidence. If a risk assessment fails to consider all relevant risks or is overly optimistic in its conclusions, the regulator may require revisions, reject the associated OMP or refuse the permit application.

A major theme throughout the new odour guidance is the concept of appropriate measures, which encompasses but is not limited to Best Available Techniques (BAT). Appropriate odour control measures must be proportionate to the risk of odour pollution and cost-effective, and should not compromise other environmental outcomes. Such measures might range from basic inventory control and process optimisation to more advanced engineering solutions such as containment systems, ventilation with abatement technology, or operational scheduling to avoid peak odour release times. The guidance emphasises that not all control measures are equally appropriate for every site, and selection should be based on the specific odour sources and operational context.

The new guidance also encourages operators to consider sector-specific technical guidance on BAT and odour control, where available, to ensure their mitigation strategies reflect current industry standards.

Where H4 often focused on dispersion modelling and numerical exposure criteria, the current Environment Agency guidance highlights community and public experience as a key component of odour impact assessment. Reports from the public about odour incidents are now a primary source of information for regulators assessing odour impacts, and the guidance outlines categories for classifying the severity of odour pollution based on how odour affects people’s behaviour, use of space, health and comfort.

This approach reinforces the principle that odour pollution is ultimately judged on its effect on communities — such as changes to behaviour, cessation of outdoor activities, complaints about health impacts or disruption to sensitive environments — rather than simply on numbers derived from models.

A central deliverable under the new guidance is a well-constructed Odour Management Plan. The OMP must describe how odour will be prevented or minimised, document the odour risk assessment, outline appropriate control measures, and detail how these measures will be implemented and monitored over time. The OMP should address all relevant site conditions, including inventory of odorous materials, containment and abatement systems, process controls, monitoring procedures, contingency plans for abnormal conditions, and procedures for investigating and documenting odour complaints. Records must be auditable and performance standards should be defined to demonstrate that control measures are effective.

Operators are expected to tailor the OMP to the scale and complexity of their activities. A site with low odour potential may require a more concise plan, whereas sites with high odour risk need comprehensive and detailed documentation.

For environmental consultants, developers and operators who previously prepared H4-style odour assessments, transitioning to the new guidance requires several key changes in approach:

• Replace numeric benchmarks with risk-based justification: Operators no longer rely on fixed odour limits like percentile thresholds. Instead, they must build a case that their odour management measures are appropriate and proportionate to prevent pollution.
• Focus on appropriate measures and BAT: Rather than treating odour modelling and sniff testing as mandatory technical exercises, practitioners should prioritise understanding odour sources and implementing proportionate control measures based on sector-specific BAT where relevant.
• Document odour risk assessments comprehensively: The new framework expects detailed risk assessment that anticipates and addresses potential odour pollution scenarios, drawing on historical data, process knowledge and monitoring evidence.
• Produce robust, auditable Odour Management Plans: OMPs must go beyond generic statements and clearly connect risk assessment outcomes with appropriate measures, monitoring protocols, contingencies and record-keeping.
• Use community and complaint data effectively: Real-world evidence, including complaints and reports of odour impacts, is now a key part of how the Environment Agency evaluates odour pollution. This means operators should incorporate complaint logs and community feedback into both assessment and management planning.