A study published in 2026 in the journal Legal Medicine highlights a reality that is sometimes underestimated at crime scenes: the presence of DNA on an object does not necessarily mean that its owner handled it. The researchers focused on a very common item in forensic investigations, the cigarette butt, to determine whether a partner’s DNA could be detected on it after a simple kiss, even if that person never smoked the cigarette. Their conclusion is unequivocal. Secondary transfer is possible, detectable, and may persist for up to two hours after contact. These findings directly challenge the interpretation of DNA mixtures in judicial contexts and reinforce the importance of evaluating evidence at the activity level, rather than solely at the source level.
A classic crime scene issue
Cigarette butts represent a major biological substrate in forensic casework. Rich in epithelial cells deposited through saliva, they generally yield exploitable genetic profiles. In practice, the discovery of a DNA mixture on a cigarette filter is often interpreted as evidence that two individuals smoked the same cigarette or handled it in close succession. However, with the increasing sensitivity of modern quantification and STR amplification techniques, laboratories are now capable of detecting minute amounts of DNA, including those resulting from indirect transfers. The question is therefore no longer simply “Whose profile is this?”, but rather “How did this DNA get there?”
An experimental protocol based on two realistic scenarios
The authors of this pilot study tested two distinct configurations.
First scenario: kiss, then cigarette.
A couple exchanged a deep kiss involving saliva transfer. Each partner then smoked a cigarette at different intervals: immediately, and then 5, 15, 30, 60, 90, and 120 minutes after contact. The objective was to assess whether the partner’s DNA, retained in the oral cavity, could be secondarily transferred onto the cigarette filter.
Second scenario: shared cigarette.
Both partners alternately smoked the same cigarette, reproducing a case of direct co-consumption. Samples were analyzed either immediately after collection or after a 30-day storage period in order to evaluate the impact of time on DNA quantity and quality.
Detectable persistence for up to two hours
In the “kiss then cigarette” scenario, alleles attributable to the non-smoking partner were detected on cigarette butts up to 120 minutes after contact. The quantity of transferred DNA gradually decreased over time but remained detectable under several experimental conditions. In other words, the presence of a minor DNA profile on a cigarette butt does not, by itself, demonstrate that two individuals smoked the cigarette. A prior intimate contact could be sufficient to explain the mixture. These findings align with previous research showing that salivary DNA can persist in the oral cavity for several tens of minutes, or longer, depending on individual and physiological factors.
The determining effect of processing delay
The study also demonstrated a significant decrease in total DNA quantity after 30 days of storage, accompanied by an increase in the degradation index. This phenomenon particularly affects the minor component of the mixture, which is more fragile and more prone to partial loss (allelic drop-out, imbalance). In practical terms, a cigarette butt analyzed promptly may reveal a detectable mixture, whereas delayed processing could result in an apparently single-source profile.
Such temporal variability complicates interpretation and underscores the importance of carefully documenting storage conditions and processing timelines.
The contribution and limits of Y-STR markers
In cases where the female partner smoked after the kiss, Y-STR analyses allowed specific monitoring of the transferred male component. Complete Y profiles were obtained up to one hour after contact, with alleles still detectable at two hours under certain conditions. However, progressive degradation and low template quantities again require caution and contextual interpretation.
Interpreting at the activity level is essential
These findings clearly illustrate the now central distinction between:
- Source level: whose DNA is it?
- Activity level: by what mechanism was it deposited?
When a mixture is detected on a cigarette butt recovered from a crime scene, several scientifically plausible scenarios may exist: co-consumption, successive handling, secondary transfer following prior intimate contact, or even a combination of these hypotheses. An expert cannot therefore limit their assessment to identifying the DNA contributors. They must also evaluate deposition mechanisms consistent with current scientific knowledge, taking into account secondary transfer dynamics and the impact of time.
Conclusion
This experimental study, conducted under controlled conditions, does not claim to establish a universal rule applicable to all judicial situations. It does, however, clearly demonstrate that secondary oral transfer onto a cigarette butt is possible and may remain detectable for up to two hours after a simple kiss. At a time when the sensitivity of genetic analysis techniques continues to increase, these results reaffirm a fundamental principle of forensic science: detecting DNA is not, in itself, proof of a particular scenario. Interpretation must be rigorous, contextualized, and grounded in activity-level reasoning in order to avoid overinterpretation before the courts..
Source :
GIANFREDA, Denise, CORRADINI, Beatrice, FERRI, Gianmarco, FERRARI, Francesca, BORCIANI, Ilaria, CECCHI, Rossana, SANTUNIONE, Anna Laura. Preliminary study of mixed traces on cigarette butts and non-self DNA transfer, persistence, prevalence and recovery in different forensic scenarios. Legal Medicine, 2026, vol. 81, article 102803. DOI: 10.1016/j.legalmed.2026.102803.
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