Oak Processionary Moth: The Hidden Hazard in Our Oaks, and the PPE and RPE Arborists Need

Every spring, a small, unassuming caterpillar reappears in the oak trees of South East England, and with it comes a genuine risk to human health, animal welfare and tree survival. The Oak Processionary Moth (Thaumetopoea processionea), or OPM, is now a familiar name to local authorities, landowners and tree-care professionals across London and the surrounding counties. For the arborists who treat and fell infested trees, it is also one of the most underestimated occupational hazards in the sector.

At Sentinel Laboratories, protecting people is personal to us. With more than 35 years advising on respiratory and personal protection across industrial, laboratory and outdoor working environments, we want to set out clearly what OPM is, the threats it poses, and exactly what PPE and RPE an arborist needs to work safely on infested trees.

What is Oak Processionary Moth?

OPM was first accidentally introduced to England in 2005 on oak trees imported from continental Europe, with the first established population identified in West London the following year. It is now regulated as a quarantine pest, subject to a government-led programme of survey and control coordinated by the Forestry Commission to limit its population and spread.

The pest is most easily recognised in its larval (caterpillar) stage. The caterpillars have black heads and grey-brown bodies covered in long white hairs, and they move nose-to-tail in distinctive processions, which is how the species earned its name. They build silken, teardrop-shaped or dome-shaped nests on the trunks and branches of oak trees, typically around the size of a tennis ball, which start white and discolour to brown over time.

The lifecycle is seasonal. Eggs are laid in late summer, caterpillars hatch from around mid-April, and they then pass through six larval stages before pupating in mid-to-late summer. The critical detail for anyone working on oaks is this: from the third larval stage onwards, the caterpillars develop their irritating hairs, making the period from roughly May to July the highest-risk window.

The threat to trees

The most visible threat is to the oaks themselves. OPM caterpillars feed on oak leaves, and large populations can strip whole trees bare. This defoliation weakens the tree, reduces its ability to photosynthesise, and leaves it far more vulnerable to other pests, diseases, and environmental stresses such as drought and flood. While healthy mature oaks can often recover from a single season of defoliation, repeated infestation places our native oak population, an iconic and ecologically vital part of the British landscape, under serious strain.

The hazards to human health

The most serious concern, and the reason OPM is so significant from a health and safety perspective, lies in the caterpillars' hairs, known technically as setae. A single mature caterpillar can carry several hundred thousand of these microscopic, barbed hairs, which contain an irritating protein called thaumetopoein.

These hairs are a problem for several reasons:

They cause a range of reactions. Contact with the hairs, whether through the skin, eyes or airways, can cause itchy skin rashes and urticaria, conjunctivitis and eye irritation, sore throats, and, less commonly, breathing difficulties and asthma-type symptoms. In sensitised individuals, reactions can be more severe.

Direct contact is not necessary. The barbed hairs detach readily and can be carried on the wind, so a person does not need to touch a caterpillar or nest to be affected. Hairs can settle on bark, foliage, grass, equipment and clothing.

The risk is persistent. Crucially for tree workers, the hairs do not lose their potency when the caterpillars move on. Old and "spent" nests remain hazardous, and shed hairs can persist in the environment for a year or more until they decay. A tree that looks quiet can still pose a real risk.

Exposure can be cumulative. Repeated contact can increase sensitivity over time, meaning that workers who are regularly exposed may experience progressively stronger reactions. This is precisely why a robust, consistent approach to protection matters so much for professionals.

The threat to pets and livestock

OPM is not only a human health issue. Curious pets such as dogs and cats are at risk if they sniff, lick or pick up caterpillars or nests, and grazing animals including horses, cattle, sheep and goats can come into contact with hairs on grass or fallen nests. Affected animals can suffer considerable distress from hairs in the mouth and nose, and anyone managing land or animals near infested oaks should keep them well clear.

The legal picture for employers

Anyone whose work brings them into contact with OPM-infested oaks needs to understand that this is not simply a matter of best practice. Where employees are exposed, the work falls squarely within the Control of Substances Hazardous to Health Regulations (COSHH) 2002. Employers have a legal duty to assess the risk, control exposure, and provide suitable protective equipment, along with information, training and health monitoring such as skin checks.

There are two further obligations worth noting. OPM nests and caterpillars must be reported to the Forestry Commission via the TreeAlert portal, and because OPM is a regulated quarantine pest, the movement of oak material is restricted within the designated management zones. Any tree work should be planned with these requirements in mind.

PPE and RPE requirements for arborists

The level of protection required depends on the task, the proximity to nests and caterpillars, and the findings of the site risk assessment. A useful way to think about it is in tiers of exposure.

Working near infested trees (lower-exposure tasks)

For general work in areas where OPM is present, but without direct disturbance of nests or caterpillars, the baseline is to cover exposed skin. That means long-sleeved tops and long trousers with cuffs tucked in, supplemented by gloves, goggles and head covering such as a balaclava. This is the minimum, and it is rarely sufficient on its own for direct treatment or removal work.

Treatment and spraying operations

Where trees are being treated with an approved insecticide or bio-pesticide, the recommended protection combines pesticide protection with OPM protection:

• Respiratory protection (RPE): a disposable filtering half mask to FFP2 or FFP3 (EN 149:2001) to protect against the airborne hairs and spray particulates. FFP3 offers the higher assigned protection factor and is the safer default where exposure is uncertain.
• Eye protection: goggles complying with EN 166, code 4 or 5, to guard against both hairs and spray.
• Body protection: a disposable, impermeable spray suit (a Type 3/4 coverall suitable for liquid chemical protection) that also blocks the caterpillars' hairs.
• Hand protection: robust, water- and chemical-resistant gloves, as used for spraying.
• Foot protection: waterproof, chemical-resistant rubber boots.

Manual nest and larvae removal (highest-exposure tasks)

This is the work that demands the most rigorous protection. When removing nests, climbing infested trees, or operating from a mobile elevating work platform (MEWP) close to nests, the Forestry Commission is unambiguous: full body protection, including complete head, face and neck cover, plus respiration equipment, is necessary. In practice, the recommended system is:

• Powered respiratory protection: a powered air-purifying respirator (PAPR) with appropriate particulate filters. A PAPR provides a higher protection factor than a disposable mask and removes the breathing resistance that makes prolonged work in a tight-fitting mask difficult, which matters during physically demanding climbing work. There are two routes here. A tight-fitting full-facepiece PAPR delivers strong protection but still depends on a good facial seal, so it requires fit testing. Alternatively, a loose-fitting powered respirator sidesteps that requirement entirely. We particularly recommend the Gentex PureFlo ESM+, a self-contained, all-in-one powered system that combines head, eye, face and respiratory protection in a single unit, with an integrated hard hat, wide-vision face shield and HEPA filtration. Because it is loose-fitting, it needs no face fit testing, can be worn by operators with beards or facial hair, and has no trailing hoses or belt-mounted blower to snag during climbing — all real advantages for arboriculture.
• Full-coverage protective suit: a disposable pesticide-grade coverall that completely covers all skin and hair, with the hood correctly deployed over any head protection. Helmets with ventilation gaps must not leave the operator exposed.
• Gloves, sealed cuffs and chemical tape: all openings sealed so that hairs cannot work their way inside the suit.
• HEPA filtration for collection: where a professional vacuum is used to remove nests and larvae, it should work to a high-efficiency particulate (HEPA) specification, with filters to H13 or H14.

A vital principle running through all of this: OPM PPE should be dedicated to OPM work and not used for other tasks. Suits can be punctured or torn during climbing, and contaminated equipment carries hairs to other sites and to the home. Correct donning, doffing and disposal procedures, supported by checklists and training, are as important as the equipment itself. Contaminated reusable clothing should be washed separately at 60°C or above, though even this is not a guarantee, and replacement is often the safer course.

Why fit testing is not optional

There is one point we would urge every arborist business to take seriously. Tight-fitting respiratory protection, including FFP3 masks and tight-fitting full-facepiece respirators, only delivers its rated protection if it forms an effective seal against the wearer's face. Under UK law, tight-fitting RPE must be face fit tested for each individual wearer. A mask that fits one operator perfectly may leak badly on another, allowing hairs straight into the airways. Facial hair breaks the seal entirely, which rules tight-fitting RPE out for bearded operators.

This is exactly why a loose-fitting powered respirator such as the Gentex PureFlo ESM+ is so well suited to OPM work: it provides a high level of protection without relying on a face seal, so no fit testing is required and facial hair is not a barrier. Where tight-fitting RPE is used, however, fit testing is not optional, and this is an area where Sentinel is genuinely able to help. As ISO 9001, ISO 14001 and ISO 45001 certified specialists, with directors who are members of the Royal Society of Chemistry and the Institute of Safety and Health, we offer both qualitative and quantitative (Portacount) respiratory fit testing, alongside expert advice on selecting the right RPE and PPE for the task. We do not simply sell protective equipment; we help you choose and validate the right protection for your specific risk.

Working safely with OPM

Oak Processionary Moth is here to stay in parts of England, and as populations spread, more tree-care professionals will encounter it. The hazards are real, but they are entirely manageable with the right knowledge, the right equipment, and a disciplined approach to protection and decontamination. Get the risk assessment right, match the PPE and RPE to the task, fit test every wearer, and treat OPM-specific equipment as exactly that.

If your team works on oaks in affected areas and you would like to talk through your respiratory protection, fit testing, or the full PPE system your operators need, our team would be glad to help. Protecting you is personal to us, and our mission is simple: to make sure you go home safely, every day.

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Sentinel Laboratories Ltd is a family-run specialist distributor of PPE, RPE and laboratory consumables, based in Lindfield, West Sussex, and serving customers across the UK since 1988.

Get in touch: +44 1444 484044 | sales@sentinel-laboratories.com | www.sentinel-laboratories.com

This article is for general guidance. Always carry out a task-specific COSHH risk assessment and refer to the latest Forestry Commission and HSE guidance before undertaking work on OPM-infested trees.