Sweat, also known as perspiration, is a bodily fluid produced by sweat glands in the skin. Sweat is mostly water, but it also contains small amounts of electrolytes, proteins, and other compounds. A common question many people have is: does sweat contain DNA?
Quick answer: Yes, sweat does contain trace amounts of DNA
When we sweat, a small number of skin cells can be picked up in the sweat and make their way to the surface of the skin. Skin cells contain DNA, therefore trace amounts of DNA can be found in sweat.
How does DNA get into sweat?
DNA is found inside the nucleus of human cells. There are two potential ways DNA can make its way into sweat:
- When sweat glands produce sweat, a few skin cells may be picked up in the sweat as it moves up through the glands to the surface of the skin.
- As sweat reaches the surface of the skin and moistens the exterior layer (stratum corneum), it may pick up some shed skin cells.
In both cases, skin cells containing DNA can become incorporated into sweat. The amount of DNA is usually very small, but modern DNA analysis techniques are sensitive enough to detect these trace amounts.
What type of DNA is found in sweat?
The DNA found in sweat comes from skin cells. Skin contains several types of cells, including:
- Keratinocytes – These make up the majority of cells in the outer layer of skin.
- Melanocytes – Cells that produce melanin, the pigment that gives skin its color.
- Langerhans cells – Immune cells found in the outer layer of skin.
- Merkel cells – Sensory cells found in the outer layer of skin.
DNA from any of these cell types may be picked up in sweat as it moves through the sweat ducts and over the surface of the skin. The most likely source is keratinocytes since they are the most abundant cell type.
How much DNA is in sweat?
Only trace amounts of DNA are present in sweat. One study found that sweat deposited onto the surface of human skin contained an average of only 35 nanograms of DNA per milliliter of sweat (1).
To put this into context, a single human cell contains about 6 picograms of DNA. So the average amount of DNA found in 1 mL of sweat represents only about 5,800 cells.
Considering the surface of the skin contains millions of cells per square centimeter, this represents only a tiny fraction of the cells on the skin’s surface.
Can sweat DNA be used for identification?
The trace amounts of DNA present in sweat make it generally unsuitable for human identification purposes in forensic investigations. Routine DNA profiling requires about 1-10 nanograms of DNA as a minimum sample (2). While it’s possible to obtain this much DNA from sweat, it would require collecting substantial volumes of sweat.
For criminal investigations, DNA is typically collected from blood, saliva, semen, skin cells, or hair roots which contain abundant amounts of DNA. Sweat is generally a poor choice for identification purposes.
Does sweat contain full or partial DNA profiles?
Each human cell contains a full copy of the person’s entire genome. However, when DNA is extracted from sweat, usually only partial DNA profiles can be obtained. This is for two reasons:
- The amount of DNA recovered is low, often below the threshold needed to generate a complete profile.
- DNA in sweat comes from dead skin cells that have begun to degrade, resulting in fragmentation of the DNA strands.
Partial DNA profiles may only reveal a few distinct points along a strand rather than full sequences. This makes identification more difficult unless a match for that partial profile is already on file in a DNA database.
Can sweat transfer DNA between people?
Yes, it is possible for sweat to facilitate a transfer of DNA from one person to another. This DNA transfer can happen through direct skin-to-skin contact.
For example, if two people shake hands, some sweat may transfer from one person’s palm to the other. Any skin cells containing DNA that are present in the sweat could then be deposited on the other person.
DNA transfer can also occur indirectly via objects. If someone touches an object and leaves behind sweat and skin cells, another person touching that object may pick up the first person’s DNA.
However, DNA transfer from sweat is considered secondary transfer (from one person, to an object, to another person). Secondary transfers result in lower amounts of recovered DNA compared to primary transfers (directly from one person to another).
Can sweat be used to collect DNA for genetic genealogy?
Genetic genealogy utilizes DNA analysis combined with traditional genealogical research to generate leads in criminal cases. This involves identifying genetic relatives by comparing crime scene DNA to commercial genealogical databases like AncestryDNA and 23andMe.
For genetic genealogy, a substantial amount of high quality DNA is needed to generate a genetic profile. Typically, samples are collected from blood, semen, or saliva. Sweat is generally not a good choice since it contains low quantities of DNA that is often degraded.
While it may be possible to obtain enough DNA from sweat for genetic genealogy in some cases, sweat is considered a suboptimal sample type for this purpose.
Does sweat contain mitochondrial DNA?
Mitochondrial DNA (mtDNA) is a small circular strand of DNA located inside mitochondria, the “powerhouses” found in most cells. Mitochondria are passed down exclusively from mother to child.
Since skin cells contain mitochondria, it is likely that mtDNA is present in shed skin cells picked up in sweat. However, studies have not specifically looked at recovering mtDNA from sweat samples. Additional research would be needed to confirm the presence and quantity of mtDNA.
Can sweat create false-positive DNA matches?
When DNA is transferred via sweat from one person to another, it can potentially create misleading DNA evidence. If the sweat DNA is deposited at a crime scene, it could generate a false genetic match.
For example, Person A shakes hands with Person B prior to committing a crime. Some of Person’s A sweat and DNA are transferred to Person B’s hand. Later that day, Person B touches a door at the crime scene, depositing Person’s A DNA.
If this DNA is found during the investigation, it may falsely implicate Person A through an indirect DNA transfer. This illustrates how important it is for investigators to account for the possibility of secondary or tertiary DNA transfers from sweat.
Can sweat create false-negatives in DNA tests?
False-negative results can occur when DNA testing fails to detect DNA that is present in a sample. In the case of sweat, a false-negative could happen if:
- Insufficient sweat is collected, resulting in too little DNA for analysis.
- The DNA in sweat is degraded and cannot be typed.
- Contaminants or inhibitors are present that interfere with DNA amplification.
The small amount and often poor quality of DNA in sweat makes false-negatives more likely compared to samples like blood or semen. Repeated testing using different sample volumes or collection methods can help minimize false-negative risks.
Does sweat need to be preserved for DNA analysis?
To maximize DNA typing from sweat, proper sample collection and preservation is important. Left untreated, the DNA in sweat will rapidly degrade due to nucleases (enzymes that break down DNA) released when the cells die.
Ideally, sweat should be air dried on sterile gauze, sealed in paper envelopes, and stored frozen. This helps prevent further cell lysis and DNA breakdown prior to testing.
Chemical preservatives can also be applied to sweat samples. Common preservatives include EDTA, SDS, and NaCl which protect against nuclease activity and bacterial growth.
Can touch DNA from sweat be recovered days later?
“Touch DNA” refers to DNA recovered from skin cells left behind when a person touches an object. In the case of sweat, touch DNA originates from skin cells present in the sweat deposit.
Several factors influence how long touch DNA from sweat can persist on an object, including:
- Surface material – Porous materials hold DNA longer than non-porous ones.
- Exposure – UV light, heat, humidity degrade DNA over time.
- Handling – Additional contact adds/removes cells.
- Initial amount – More starting material persists longer.
One study found usable full DNA profiles could be recovered from fabric items even after 1 week at room temperature (3). However, recovery rate dropped substantially after 3 days.
While possible to recover touch DNA days later, the chances of obtaining a complete genetic profile decrease over time as DNA degrades.
Can DNA be extracted from ancient or old sweat samples?
DNA has been recovered from a wide range of ancient biological samples including bones, teeth, hair, and preserved tissues. However, recovering DNA from aged sweat samples is challenging.
Compared to hard tissues like bones or teeth, sweat provides relatively poor DNA preservation. Skin cells have minimal structural integrity compared to mineralized or keratinous tissues. Also, sweat contains nucleases and bacteria that rapidly degrade DNA.
While an occasional full DNA profile has been recovered from aged fingerprints, in most cases sweat DNA degrades too quickly for analysis after more than a few weeks of storage. Significant advances in extraction techniques would be needed to reliably obtain DNA from older sweat stains.
Conclusion
In summary, sweat does contain trace amounts of DNA originating from skin cells picked up as sweat is produced and released onto the surface of the skin. However, the quantity is generally quite low and DNA quality is often degraded compared to other sample types. Sweat DNA can be used for identification purposes but works best when fresh and properly preserved. It is also possible for sweat to transfer DNA from one person to another or to objects.
References
- Park, S.J., Kwon, O.H., Kim, J.H., Yoo, H.S., Lee, H.C., Woo, K.M., Kim, S.Y., Lee, S.H. and Kim, Y.S., 2014. Ultra-trace forensic detection of sweat by molecular aptamer beacons. Analyst, 139(21), pp.5457-5463.
- Butler, J.M., 2010. Fundamentals of forensic DNA typing. Academic Press.
- Raymond, J.J., van Oorschot, R.A., Gunn, P.R., Walsh, S.J. and Roux, C., 2009. Trace DNA success rates relating to volume crime scenes: A pilot study. Forensic Science International: Genetics Supplement Series, 2(1), pp.136-137.
| DNA present in sweat | Yes (in trace amounts) |
|---|---|
| Main source of DNA | Shed skin cells |
| DNA quality | Partial and degraded |
| Possible uses | Limited identification, DNA transfer analysis |
| Ideal storage | Air dried, frozen |
| Persistence over time | Degrades within days to weeks |
In conclusion, while sweat does contain small amounts of DNA, it is generally not a optimal source material for most forensic identification purposes due to low quantities and degraded quality. However, analysis of sweat DNA can provide supporting evidence in some cases, especially when evaluating DNA transfer events.