Use of Wipe Sampling to Assess Surface Contamination

What is Wipe Sampling?

Wipe sampling is a technique that can be used to assess the quantities of a chemical (or other contaminants) on a surface. It does not measure individual exposure or directly evaluates health risks; instead, it is a tool to help establish the suitability of cleaning techniques or determine the level of contamination on surfaces from hazardous materials.

When is Wipe Sampling Used?

Contamination of work surfaces, such as equipment, door handles, handrails, and furniture, with chemicals may contribute to overall employee exposure through local skin contact, skin absorption, or ingestion. Wipe sampling can help investigate the level of exposure from these surfaces. A decision to use wipe sampling as an investigative tool or for routine monitoring should be based on a chemical risk assessment.  

It is likely to be of value when:

• Non-volatile materials are being handled that can be absorbed through the skin or can act directly on the skin.
• The chemical risk assessment indicates a potential for skin contact and that wipe sampling is necessary to confirm that there are no unacceptable dermal exposures – this is most likely where highly potent materials are handled.
• There is a need to verify that engineering controls, work practices, and administrative controls meet the intended performance criteria.
• It is necessary to confirm the effectiveness of cleaning or decontamination procedures where skin absorption or ingestion is likely to be a significant route of exposure.
• There is potential for skin irritation or sensitization in non-process areas where people may not be protected by Personal Protective Equipment (PPE) or working practices.

Additional applications for wipe sampling include the following:

• In production and non-process areas for assessing the cleanliness of surfaces, e.g., in facilities where physical segregation is made between process and non-process areas to prevent the spread of contamination.
• When a semi-quantitative method is needed to check that equipment has been adequately decontaminated.
• It is dismantled for maintenance work.
• Final disposal if it is worn out.
• It is returned if it is leased (this is especially important when ownership or control of equipment is to be transferred to another individual or organization).

Where is Wipe Sampling Performed?

Selection of appropriate areas for wipe sampling is made after consultation with workers and study of typical work activities. For surface cleanliness studies, sample locations may be conveniently divided into process and non-process areas. Non-process areas should not be neglected since any spread of contamination into these areas is more likely to result in exposure due to removing protective clothing.  

Examples of specific process areas are:

• Handrails on staircases
• Valve handles (pipework, sample points, etc.)
• Adjustable Local Exhaust Ventilation (LEV) hoods or dampers
• Door handles
• Plant telephone sets
• Desk surfaces
• Fume hoods
• Laboratory benches

Examples of specific process areas are:

• Desktops in the control room
• PPE store
• Lockers in changing room
• Floors in the shower area
• Door handles, tables, and light switches in lavatories and washrooms

How is Wipe Sampling Performed?

The wipe sampling method consists of wiping the selected surface locations using a collection medium (usually a filter paper) which is subsequently analyzed using the occupational hygiene sampling and the analytical procedure recommended for the material concerned.

The procedure for routine sample collection, using either filter paper or cotton wool, is described below:

1. First, the collection medium to be used is selected.  A small glass-fiber filter is usually suitable.  The collection medium needs to be wetted for sampling with a solvent appropriate for the substance and analytical procedure. 
2. A pair of clean, disposable gloves are used when handling the sampling medium to prevent the risk of cross-contamination between samples.  A pair of clean tweezers may also be used when handling cotton wool.  When used for sampling very potent compounds, which may be present in small quantities, the tweezers may also be disposable.
3. The sampling medium is wiped across the selected area, and the swab is drawn across the area in a zigzag fashion.  Enough pressure is applied to remove easily dislodged residues from the surface without damaging the sampling medium.  
4. The sample is placed in a clean, impervious, labeled container and sealed with a cap.
5. A note is made of the exact location, date, surface condition (including any visible signs of contamination), and other relevant information for each sample.
6. The sample is sent to the laboratory for analysis.

How Many Wipe Samples Should be Taken?

The number of samples taken should be sufficient to evaluate the cleanliness of all surfaces identified in the sampling plan.  The exact number will depend on the size of the area; as a rough guide, it might range from 3–4 samples in a small locker room to 20–30 samples in a large process area or laboratory.  A balance needs to be established between covering important locations and creating an excessive sampling and analytical workload.

How Often Should Wipe Sampling be Performed?

Wipe sampling is most commonly used as a tool for investigating problems.  It is rarely used for routine monitoring because of the lack of established standards and the difficulty of obtaining meaningful and reproducible results.  However, it can be used routinely to look at trends over a period of time for a particular location.  In such cases, the chemical risk assessment should indicate how often routine monitoring might be required in the light of factors such as:

• How quickly dust deposits appear on surfaces
• Whether the material concerned is a potential skin irritant
• Whether the material is readily absorbed through the skin
• The likely severity of effects if over-exposure should occur
• How long would it take to rectify a deficiency once identified

Sources:

1. Kibby, T. A review of surface wipe sampling compared to biologic monitoring for occupational exposure to antineoplastic drugs. Journal of Occupational and Environmental Hygiene. March 2017

2. Simplivia, Using Wipe Sampling to Detect Contamination

3. OSHA. Evaluation Guidelines for Surface Sampling Methods.