Sweat and Sebum Trace Analysis: Myths and Realities

Numerous domestic TV shows, like the series “Sled” and similar productions, have irreversibly cluttered the minds of the general public in the former CIS countries. Aiming to instill the idea of the inevitability of punishment and the certainty of identifying a criminal by a flake of dandruff left a kilometer from the crime scene, scriptwriters often disregard accuracy when it comes to forensic examinations in criminal proceedings. Today, let’s talk specifically about the analysis of sweat and sebum traces (SST)—one of the most common types of forensic analysis in modern practice.

First, a disclaimer: the author of this article is neither a forensic expert nor a medical professional and, for objective reasons, cannot explain the specific methodologies used in these analyses. However, experience in ordering forensic examinations and analyzing their results allows the author to reliably discuss the main points that may be relevant for potential clients of law enforcement agencies.

Understanding the Terminology

Let’s start with the terminology. Sweat and sebum substance (SSS), which forms the basis of a sweat and sebum trace, consists of the following components: sweat (mainly water), sebum (a product of sebaceous gland secretion), keratinous fat (a product of the natural skin renewal process), and a range of trace microelements. SST, a broader term, also includes the skin microflora of the person who left the trace on a surface, as well as specific characteristics of how the trace was made (such as smudging, abundance, etc.).

What Can Forensic Experts Learn from SST?

1. Fingerprints and Palm Prints
   The most well-known aspect is, of course, the infamous “fingerprints,” which in practice can include prints from hands (fingers and palms) and feet. Everyone knows about the uniqueness of papillary patterns, but it’s worth noting that even partial palm prints (without fingers) can be used for identification. A practical tip: after wiping your hands with a solvent, it’s nearly impossible to leave traces on surfaces. Another method is treating palms with BF-6 medical glue, commonly used for wound care. It forms a protective film on the skin in minutes, preventing trace formation. This is especially useful for people who need fine motor skills and can’t wear gloves.

   However, often the amount of material is insufficient for reliable fingerprint analysis—the contact area may be too small, or the print may be smudged. Contrary to what you see in TV shows, it’s usually impossible to create a clear image from a smudged print and match it triumphantly to a database. For reference, the fingerprint database in Russia is called ADIS Papillon.

   Fun fact: Papillary patterns themselves, regardless of the SSS that formed them, can also provide information. For example, people with Down syndrome have only one flexion crease on the pinky (instead of two), and a single transverse (sometimes called “simian”) crease on the palm. Certain features of papillary lines can indicate congenital heart defects. In one real case, this sign helped narrow down suspects in a rural area by reviewing medical records, and the person was identified within a week.

   Papillary lines can also help estimate the age of the person who left the print, though this is always somewhat approximate. For example, an 8-year-old child typically has 12-13 lines per 5 mm, while an adult has 8-10. There are more complex indicators, but generally, it’s possible to estimate age from a full palm print with an accuracy of 10-15 years. There are also borderline-scientific studies attempting to describe mental characteristics based on papillary patterns, but these remain speculative.

2. Analysis of Sweat and Sebum Substance (SSS)
   The first thing Russian forensics learned to determine from SSS is the gender of the person who left it. Interestingly, not all law enforcement officers are aware of this, and the author has seen cases where this question wasn’t even posed to the expert. Sometimes, based on the amount and concentration of sweat on clothing or other items, an expert can even determine which phase of the menstrual cycle a woman was in when she left the trace.

   The composition of sweat also varies depending on the cause of sweating. For example, during intense physical activity or high temperatures, sweat is mostly water, while fear-induced sweating has a higher concentration of solids. This can sometimes have evidentiary value.

   SSS allows for a much more accurate determination of the age of the person who left the material, primarily due to differences in lipid composition by age and gender. Modern methods can narrow down age identification based on these indicators.

   SSS analysis can also indicate, with reasonable probability, whether the person had diabetes, obesity, acne, or psoriasis—helpful for narrowing down suspects.

   Since SSS is highly individualized, in some cases it’s possible to determine whether the person who left the SST had recently used drugs. However, with current methods, only heroin, cocaine, or morphine use can be reliably detected, and only within 1-2 days of use. If the trace was left during this peak period, the drug can sometimes be detected even weeks later.

   SSS can also reveal traces of perfumes, indicating whether the person was wearing fragrance at the time. It’s generally impossible to identify the exact product, as their compositions are very similar, but some distinctions can be made between perfumes (eau de toilette) and deodorants (which contain aluminum salts), or aftershave (which may contain menthol).

   It’s important to note that SSTs can be left not only by hands or feet, but by any part of the body. Besides extremities, traces are often left by cheeks, forehead, forearm, bare stomach and back, buttocks, and thighs. The latter is especially common in cases of spontaneous, domestic homicides.

3. DNA Identification from SST
   SST contains “keratinous fat,” a product of natural skin renewal, which makes DNA identification possible. However, SSS contains genetic material (DNA) only in the skin cells present in it. This means that a usable concentration of DNA is found only where skin cells are most frequently renewed due to mechanical impact—mainly in traces left by hands. Even then, sufficient DNA is found in only about 50-55% of cases. For traces left by other body parts, this drops to 15-20%. For women’s faces covered with makeup, the chance is only 3-5%, barely above statistical error.

   In practice, this means that a drop of sweat on the floor, for example, is not enough to identify a person. However, sweat that has soaked into clothing, combined with skin cells inevitably present on the fabric, can serve as a DNA source if the clothing is collected as evidence.

   There’s also a practical consideration: DNA analysis is still expensive, so its use is strictly regulated. In murder cases, DNA analysis is almost always ordered if any material is available. In theft cases, however, it’s highly unlikely due to budget constraints, and regional offices often run out of funds for such tests by the end of the third quarter.