Axillary Malodor Efficacy

Generally, axillary malodor efficacy tests are conducted on three product types: antiperspi-rants, deodorants, and soaps. Although antiperspirants are primarily designed to inhibit sweat production, they are also considered deodorants because they inhibit sweat, which acts as a culture medium for bacteria to produce, degrade, and form malodor. Deodorants are formulated to control malodor only, through absorption, fragrance masking, and/or by reducing antibacterial activity. Some soap products may also reduce axillary malodor by fragrance masking and/or inhibiting bacterial growth.

The use of sensory testing applications is the primary methodology used to establish deodorant efficacy. In 1987, the sensory testing division of The American Society for Testing and Materials (ASTM) published the Standard Practice for the Sensory Evaluation of Axillary Deodorancy [2]. This document recommends that a product meet the criteria presented in the standard in order for it to qualify as an effective deodorant.

The basic design for conducting a deodorant study consists of selecting subjects with high axillary malodor, applying or using the test material at selected intervals, and then measuring the level of axillary malodor using a panel of trained odor judges. The test material is considered effective if there is a statistical difference between it and a placebo or untreated control. Factors that are critical to the test design and consequently a successful study are the subject selection, subject restrictions, odor-judge selection and training, selection of a suitable test location, and using appropriate scaling techniques.

Subjects should be selected from the user population and have a distinct axillary odor. Those with extremely high or low odor and those with large differences in odor level between the right and left axillae are usually disqualified. It is important that potential subjects participate in a washout or conditioning period before selection to prevent a carryover effect from the use of other products. For a minimum of 7 days, subjects are not allowed to use any axillary products and are instructed to wash the axillae only with a mild, nondeodorant soap. Other restrictions that are known to interfere with sensory odor assessments require the subjects to abstain from swimming, excessive exercise, and from using any fragranced products. They must also avoid spicy foods, and before an odor evaluation they are restricted from smoking.

Perhaps the most crucial factor in a well-executed malodor efficacy test is the selection and training of qualified odor judges. This process requires management support and commitment, a sensory staff or analyst to conduct the training, and a pool of available and interested candidates. Other factors to consider are the time commitments to not only select and train the judges but to continually maintain and validate their performance. The odor-judge selection and training process involves four basic steps: 1) interviewing candidates, 2) conducting screening tests, 3) training, and 4) validating performance.

During the interviewing process, candidates who have conflicting commitments or interfering health problems should be discontinued. A description of the test and the odor-judge process must be explained to each individual. If possible, because of the unusual nature of the intended task—sniffing the axillary region of subjects—a video of the process should be shown. Through interaction and discussion, those candidates who show a sincere interest and are willing to commit to the program are identified.

In vitro screening tests are administered to the potential odor judges to determine their olfactory acuity and ability to discriminate and reproduce results. Because it is possible for some individuals to be insensitive to some of the odors generated by the human body, potential judges should also be screened for this inherent lack of sensitivity.

An inability to recognize some body odors is commonly referred to as ''specific anosmia.'' The anosmias that have been identified in axillary odor include sweaty, urinous, musky, and hircine odors, with the primary anosmia being a urinous smell. It has been reported that as many as 46 to 50% of the population are insensitive to the urinous odor [3]. Because of this high percentage, judges should be screened for the insensitivity using the odorant androstenone. Individuals who can smell this compound will rate it extremely strong and often find it offensive, whereas those who are anosmic will rate it low or may not smell it at all.

The odorant used most often to represent a sweaty smell is isovaleric acid. It is therefore often used in odor, judge acuity screening tests. Potential judges are often given a series of paired comparisons and at least one ranking test of the five established levels of isovaleric acid (Table 1) [4].

For the paired comparison tests, potential judges are given at least eight different combinations of concentrations. The pairs should represent different levels of difficulty between samples, e.g., 0.013 versus 0.87 and 0.053 versus 0.22. In the ranking test, a sample of each concentration is presented. Before administering the tests, the samples should be placed in identical bottles or jars. Each bottle is identified by a unique three-digit number. The pairs and ranking test should be randomly presented to the candidates with a distinct rest period between each test. When presented with each pair, the odor-judge trainee is asked to identify which sample has the stronger or more intense odor. For the ranking test, they rank the samples from the least to the most intense odor. It is always very important to control the conditions of the test area when administering any sensory test [5].

In addition to determining acuity, reproducibility should also be considered. This can be accomplished by administering the same tests one or two more times on separate days. The order of set presentation, bottle order, and coding system must be changed between days.

Training is initiated once individuals who show a high olfactory acuity and consistency are identified. Several steps are involved in the training process, including establishing a standard method for evaluating, identifying judge restrictions, providing reference standards that represent the scale, and conducting training sessions.

The method frequently used to evaluate the axillary region involves placing the nose near the surface of the skin located in the center of the axilla and taking several short bunny sniffs. Judges clear the sinuses by breathing into a cotton material or toweling between evaluations. The evaluation method should also include an established rest period between evaluations and/or subjects (e.g., 30 or 60 sec). The judges must also avoid touching the subject with either their nose or hands. In addition to avoiding contact, the

Table 1 Five Established Levels of Isovaleric Acid

Odor level

Concentration of aqueous solution of isovaleric acid (mL/L)









Very strong


judges are restricted from wearing any personal products with a distinct fragrance. They should also be restricted from eating certain foods before evaluating.

Critical in the evaluation process is identifying a scale. To evaluate the intensity or express the degree to which axillary odor is present, two types of scales are usually considered. One is a line scale, which consists of a standard-length line on which the judge makes a mark. The primary disadvantage to this approach is that judges may have difficulty establishing consistency without a number to remember [6]. Category scale methods are perhaps the most frequently used. This type of scale involves using sets of words and/or numbers to identify established intervals on the scale.

Among the available category scales, a 0-to-10 numerical scale has been used to evaluate or score malodor intensity. Although some descriptive language may vary slightly, the zero on this scale consistently represents no malodor while the 10 represents extremely strong malodor. Table 2 is a complete example of a 0-to-10 numerical scale.

In addition to being used in judge-acuity screening tests, isovaleric acid is often used as a reference standard when training judges to use malodor intensity scales. The five concentrations previously identified can be used to represent various points on the selected scale or other concentrations can be used. After introducing the reference points, the judges should practice until they can repeatedly assign the correct score to each reference under blind conditions.

New judges being introduced to human axillary odors should, if available, train with an experienced judge. The new judge observes the score given to a certain subject then evaluates the same subject. After participating in this capacity for a period, the judge in training evaluates the subject first and then observes the scores given by the established judge. Finally, the judge trainee evaluates independently until statistical analyses of his/ her data correlates with the established judges.

Training new judges without the benefit of established judges can be accomplished by using a couple of different approaches. In one approach, the sensory scientist training the group can determine the odor level of selected subjects then introduce the new judges to these odor levels using the previously discussed techniques. Another approach allows the new group of judges to standardize their scores through consensus. After each evaluation, the group discusses their scores, and repeats the process until they agree on the odor level for that subject. This process is repeated until independent evaluations correlate.

Table 2 A 0-to-10 Numerical Scale

Numerical value Description of malodor

Table 2 A 0-to-10 Numerical Scale

Numerical value Description of malodor


None, no malodor


Threshold malodor


Very slight malodor


Slight malodor


Slight to moderate malodor


Moderate malodor


Slightly strong malodor


Moderately strong malodor


Strong malodor


Very strong malodor


Extremely strong malodor

Although this approach is more time consuming, it often establishes a strong sense of commitment and involvement in the process for the new judges.

Once established, odor judges can be used to evaluate any personal-care product used in the axillae to control malodor. By using combinations of subject selection, product-treatment techniques, post-treatment evaluation times, and controlling environmental conditions, an almost endless number of possibilities can be evaluated by the judges. In addition to directly evaluating human subjects, odor judges can also be used to evaluate axillary odor that has been transferred to some other medium such as a t-shirt or a cloth worn against the axilla.

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