Odor-absorbing dressings differ in lab studies

Dressings use activated charcoal to absorb odors

Odors produced by chronic wounds present a real problem for wound care professionals, patients, and family members. At times, odors can become so severe as to cause a patient to withdraw completely from social interactions, even with family and close friends.

Within the category of nonmalignant chronic wounds, leg ulcers are most commonly associated with noxious odors. The smell results from an amalgam of volatile chemicals including short-chain organic acids produced by anaerobic bacteria and by-products of proteolytic bacteria, according to Stephen Thomas, PhD, director of the Surgical Materials Testing Laboratory in Bridgend, Wales, UK. Thomas has conducted research on the properties of odor-absorbing dressings. Some research even suggests that a particular wound odor can be used to identify the specific bacteria present, which can be confirmed through lab testing, he notes.

Ways to reduce wound odor

Thomas says the best way to handle odoriferous wounds is to prevent or eliminate the infection causing the smell. This, of course, may involve systemic antibiotics or antimicrobial agents. These measures are not always successful due to the nature of the wound, particularly when large amounts of necrotic tissue are present.

One potentially successful option for reducing wound odor is to use a hydrogel that contains metronidazole (about 0.8%). Thomas says this combination has been found to have an effect on a range of aerobic organisms. However, the usefulness of this approach has been questioned.

Other, less conventional approaches to reducing wound odor include the use of honey and sugar, which are thought to produce an environment that inhibits bacterial growth and therefore prevents odor formation. Even live yogurt cultures have been applied to fetid wounds to encourage an exaggerated growth of pathogenic organisms by lactic acid bacteria. Larval therapy (maggots) also has been used to mitigate wound infection and decrease necrotic wound odors.

Historically, wound odors were masked by burning incense, and in more recent times, by the use of aerosol sprays or air fresheners. Although these do not resolve the underlying problem, they may make life a little more bearable for patients and families.

Wound odor cannot always be prevented, and most dressings are not designed to absorb the molecules responsible for creating the smell. But within the past couple of decades, special odor-reducing dressings have been developed, all of which contain some form of activated charcoal, a material well-known for its ability to absorb odors. It is believed that the molecules responsible for production of the odor are attracted to the surface of the carbon and are held there by electrical forces. Most of these molecules are small and can be detected by the nose in low concentrations in the air. A single dressing, by virtue of the large surface area of the carbon, is capable of absorbing a very large number of molecules and should therefore be able to eliminate wound odor for extended periods.

Several odor-absorbing wound dressings are currently on the market. The first, Actisorb, arrived in the mid-1970s. Thomas says lab studies showed that bacteria attached firmly to its charcoal fabric and were removed from solution in the wound exudate; the bacteria, however, remained viable. A second-generation dressing, Actisorb Plus, contains 0.15% silver chemically bound to the carbon. Silver is an antimicrobial capable of killing bacteria absorbed by the dressing. (See related stories on silver dressings, pp. 13-16.)

Odor-reducing dressings can be placed directly on wounds and may be held in place by secondary dressings placed over the primary layers. Sometimes a retaining bandage is used to hold the assemblage in place.

Information lacking on odor-absorption capabilities

Thomas conducted a laboratory study comparing the odor-absorbing capabilities of four such dressings.1 The results, he suggests, may have clinical as well as esthetic implications for wound care.

Thomas sought to determine whether these dressings varied in their abilities to handle wound fluids and to control exudate odor. He notes that "despite the relatively widespread use of odor absorbing dressings . . . little objective comparative data is available on their odor and fluid handling characteristics."

Several investigators have tested the efficacy of odor-absorbing dressings of both chemical and biological materials. Chemical techniques often are favored because the efficiency of the dressing can be determined using standard analytical techniques such as gas liquid chromatography. Determination of dressing performance using biological materials is restricted to more subjective methods of assessment, such as the use of a human test panel. In one example of a biological assessment, researchers compared the odor-absorbing properties of five dressings containing activated charcoal with that of a cotton gauze swab as a control. A panel of volunteers was asked to assess the odor liberated from test samples after the addition of cultures of bacteria isolated from malodorous wounds.

The aim of Thomas’ study was "to develop a more objective test system that could be used to compare the ability of different dressings to prevent the passage of a volatile amine when applied to a wound model under simulated in-use’ conditions."

In the study, Thomas selected odor-absorbing dressings that also are intended as primary wound dressings: Actisorb Plus (Johnson & Johnson), CarboFlex (ConvaTec), Carbonet (Smith & Nephew), and Lyofoam C (Seton Healthcare).

Following is a brief description of each:

Actisorb Plus is made of a charcoal cloth comprising mostly carbon that is integrated into a rayon fabric. The dressing is designed for direct placement on a wound surface and to be covered by an absorbent second dressing. The manufacturer asserts that the proximity of the charcoal to the wound not only reduces the odor but also removes toxins present in the wound fluid, says Thomas. Silver ions contained in the dressing may reduce the risk of infection.

CarboFlex is a multicomponent dressing that consists of alginate and cellulose fibers bonded to a plastic film that allows liquid to travel only in one direction. Charcoal cloth and an absorbent layer are behind the film, followed by a second layer of perforated plastic. The dressing can be placed directly on the wound surface.

Carbonet boasts both fluid- and odor-absorbing properties. It contains a layer of activated charcoal cloth sandwiched between two polyethylene net layers and may be placed directly on a wound surface.

Lyofoam C consists of two pieces of polyurethane foam that enclose a fabric impregnated with activated carbon granules. It also may be placed directly on a wound.

A fifth dressing, Release, which contains no charcoal and is not intended as an odor absorber, was used as a control.

During the test, each dressing was mounted in a specially designed airtight apparatus and exposed to a test solution consisting of sodium/calcium chloride containing sodium and calcium ions at levels commensurate with those found in wound exudate. The mixture was completed with 2% diethylamine and 10% newborn bovine serum.

The test solution was applied to each dressing at a rate of 30 ml/hour, and the concentration was monitored and recorded constantly. Testing of a dressing ended when the concentration of diethylamine present in the air above the dressing rose to approximately 15 ppm, but test results were based on the endpoint of 10 ppm. Each dressing was tested six times.

The following is a summary of the results. The greater the volume of test solution introduced to the chamber before the endpoint of 10 ppm was reached, the greater was the dressing’s ability to absorb odors:

Mean Volume of Solution 
in Chamber at Endpoint (ml)
Actisorb Plus

The time taken for the concentration of diethylamine to increase by 10 ppm above baseline values was obtained for each dressing, and the volume of fluid that had been applied to each dressing was calculated.

Clear differences are demonstrated

Thomas writes: "The results of this study demonstrate clear differences in the ability of the products to contain the test solution and prevent the loss of the volatile diethylamine into the surrounding air. Release, an absorbent dressing which contains no activated charcoal, is able to delay the passage of the diethylamine through the dressing but is less effective in this regard than Actisorb Plus, which has limited absorbency. This suggests that the odor absorbing properties of the dressing are determined by at least two factors, the physical absorbency — a function of the presence of some form of absorbent layer, and the activity of the charcoal cloth itself. Products which combine a physical absorbent with a charcoal component show enhanced performance, as might be anticipated."

The results, he adds, do not show if odor absorption of a dressing is most effective when the dressing is applied directly to the surface of a wound or when it is incorporated into the structure of the pad or used as a secondary dressing. Further study is required to determine if the performance of a particular product is impaired once the dressing is saturated with wound fluid, says Thomas.

Based on the test results, the dressings should be expected to provide some degree of odor control from 12 hours to three days, depending on the amount of wound exudate.

Thomas notes that the rate at which the test solution was applied during the test was "considerably in excess of that encountered clinically." This may effect the performance of the dressings, though the variation probably wouldn’t affect the rank order of the products because the test is comparative.

Thomas concludes that despite some of his own criticisms of the methods and procedures he used during the investigation, the results provide, "possibly for the first time, an objective method for comparing the performance of different odor absorbing dressings when challenged with a test solution containing an odiferous volatile amine under simulated conditions of use."


1. Thomas S. Odor absorbing dressings: A comparative laboratory study. World Wide Wounds 1998; http://www.smtl.co.uk/World-Wide-Wounds/.