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8. Occupational health – OPM Manual

Oak Processionary Moth (Thaumetopoea processionea) caterpillars are covered with thousands of minute, irritating hairs which can be released as a defence mechanism or blown off by the wind.

The caterpillars build communal nests on the trunks and branches of oak trees to protect themselves from predators. Once released, the hairs can persist in the nests and the environment for a long time, posing a long-term nuisance to people and animals, such as horses and dogs, unfortunate enough to come into contact with them.

To protect public, animal and tree health, the Government encourages owners of infested trees to engage tree professionals to treat them. These workers, as well as others who work on or close to oak trees in the affected areas, are exposed to an occupational health risk which must be managed.

Effects of exposure

Contact with the hairs, which contain an urticating (irritating) substance called thaumetopoein, is known to cause a number of conditions, including:

  • skin irritation;
  • eye complaints; and
  • respiratory effects.

To date there have been no conclusive reports of OPM contact causing anaphylaxis.

Occupational risk

Landscapers, arborists, forestry workers, tree surgeons, gardeners and ground-care professionals are at the greatest risk of exposure because of the nature of their work, particularly when working on or around infested oak trees in areas where the pest is present. (See the map at These workers should take particular care to protect themselves when working with or close to nests or caterpillars, because these hold the highest numbers of hairs.
Some people can become sensitised by repeated exposure to the hairs, meaning that the symptoms become worse with repeated exposures. Therefore workers and managers must be vigilant and employ an adequate occupational health monitoring system.

Prevention and treatment

Before starting operations, consider the likelihood and risk of exposure to OPM and any mitigation which might be required, for example:

  • Is the work area within or near a known OPM-infested area?
  • Is there site-specific information about OPM presence?
  • Will you be working on host trees (oaks), or are host trees present on site?
  • Is there evidence of OPM infestation on or near the trees you are working on?

When working in areas where OPM is known to be present:

  • avoid direct contact with caterpillars and their nests; and
  • wear long-sleeved tops and long trousers with the cuffs tucked in.
  • If OPM nests or caterpillars are found in a tree undergoing arboricultural work, the work should be stopped as soon as the tree has been made safe. The area should be signed and cordoned off to warn others that OPM is present. The finding should be reported to the local authority or the Forestry Commission via Tree Alert ( Work should not recommence until the nest(s) and larvae have been appropriately removed and destroyed.

The Personal Protective Equipment (PPE) required for those involved in nest removal is:

PPE Protection Standard
Face mask To prevent inhalation Filtering half mask FFP2 or FFP3 (European standard EN149: 2001), disposable, to protect against particles
Goggles To protect eyes Goggles complying with European standard EN166 and either code 4 or code 5
Disposable spray suit To prevent skin contact An impermeable protective suit suitable for insecticide spraying will also protect against the larval hairs
Gloves To prevent skin contact Robust water- and chemical-resistant gloves, as used for spraying operations
Boots To prevent skin contact Water-proof and chemical-resistant rubber boots, as used for spraying operations
Climbing equipment for reaching nests high in a tree To prevent falling Ropes and harnesses used for climbing can retain hairs and, therefore, should only be handled with protective gloves. They should also be bagged after use and maintained solely for the purposes of removing nests

All of these items are readily available from good safety equipment suppliers.

Employers’ responsibilities

Employers have certain responsibilities for their own and their employees’ safety. Their legal responsibilities are covered by the following legislation:

In general terms employers are responsible for:

  • undertaking risk assessments;
  • identifying and enacting control measures, and ensuring these are used, for example, staff awareness of OPM, how to identify nests and caterpillars, and what to do if it is discovered;
  • providing their employees with suitable PPE; 
  • having a system for employees to report defective equipment;
  • providing their employees with information and training on the health risks and control measures, including skin checks;
  • providing suitable welfare facilities, such as washing facilities; and
  • providing a health surveillance programme.

Employees’ responsibilities

Employees should:

  • take reasonable care of their own health at work;
  • follow recommended systems of work;
  • use the equipment, including the PPE, which their employers have provided;
  • report to their employers any defects or problems with the equipment;
  • attend any training or instruction provided by their employers;
  • inform their employers if they have concerns about their health;
  • attend any health surveillance programme or health checks which employers have put in place; and
  • consult a GP if their employers have any concerns about their health after working near OPM.

If in doubt, always consult a GP about OPM-related health incidents.

Suggested treatments

For dermal (skin) symptoms

  • Calamine lotion or other similar creams can be used to relieve itching.
  • Corticosteroid cream might be recommended by medical practitioners when itching has been severe or prolonged.
  • Medical practitioners might also recommend the use of oral antihistamine medicines (tablets, capsules or syrup) where appropriate, although dermal antihistamines (skin creams) are not advised because of their potential to sensitise.

For ocular (eye) symptoms

  • If eyes are suspected to have been exposed to the hairs, the affected person should be referred to a GP for advice and treatment immediately.

For oral and respiratory (mouth, throat and nose) symptoms

  • If oral and/or respiratory system exposure is suspected, the employee should be referred to a GP for advice and treatment.

Further information

Last updated: 11th July 2017