Introduction

Carbon dioxide (CO2) is a colorless and odorless gas which has gained increased media and academic attention due to recent climate change and global warming issues. Despite its negative impact on the atmosphere, CO2 has many useful and vital properties; e.g., CO2 is used in fire extinguishers and plant fertilizers, as well as for saturation in foodstuffs. Pure CO2 supports the photosynthesis in aquarium plants, creating a natural environment for fish.

High CO2 concentrations lead to symptoms of intoxication because of the immediate toxic effect of the gas and the reduction in oxygen (O2) levels [1, 2]. Among the first signs of CO2 intoxication are tachypnea, tachyarrhythmia, restlessness, and anxiety. These are followed by neurological, with increasingly drowsiness, known as “CO2 narcosis” [2–4].

It is almost impossible to detect the gas because of its colorless and odorless properties. Carbon dioxide accumulates in high concentrations at floor level in closed rooms as it is denser than oxygen (O2) and nitrogen (N2) [5].

Fatal CO2 intoxication is rare at autopsy, and when seen are mostly due to accidents or suicides. However, these occurrences may be more common than previously thought, particularly when combined with other types of intoxication [6]. Carbon dioxide poisoning in homicides are extremely rare. The following case demonstrates the use of CO2 as the lethal tool in the murder-suicide of a family.

Case history

Four bodies of a family of five were discovered in their apartment during one of the hottest weeks of midsummer 2012 in Berlin, Germany. The bodies were those of a father (a 69-year-old business consultant), his wife (aged 26-years), and their two sons (aged 3 and 6 years respectively). The daughter, who was almost 1-year of age was not present. Two typed letters, both signed by the husband, were discovered inside the apartment. In the letters he stated that the reasons for the murder-suicide were business/financial problems, and he also explained what happened in the 36 h prior to the discovery of the bodies. He killed his wife first and then their two sons. He believed that his 1-year-old daughter had not yet developed a close familial bond and had decided to place her in a “Babyklappe” (baby hatch: permitting the anonymous deposition of an infant in a safe environment in cases were a parent cannot cope) of a hospital in Berlin. Finally, he committed suicide by CO2 poisoning. Investigations discovered that a female infant had been deposited in a baby hatch in one of the Berlin hospitals a day before the bodies were found. A clinical forensic examination showed that the infant girl was healthy and without any injuries.

Crime scene

There was no evidence of a violent struggle at the crime scene. The wife and two sons were lying in their beds, covered with a duvet, with a single rose inside a vase close to the head of each individual (Fig. 1). External examination of the bodies at the scene did not reveal any obvious injuries or defensive marks. The bodies of both children showed early putrefaction, while the mother’s body showed more pronounced putrefactive changes, particularly to the upper part of the body; however, rigor mortis was still relatively pronounced in the lower part of her body. The husband was discovered in a supine position on the floor, situated beside a provisional sleeping arrangement; his head was covered with a plastic bag, which was tied loosely around the neck with a rope (Fig. 2).

Fig. 1
figure 1

Top bodies of the two sons at the death scene. Note the putrefactive changes and the roses placed inside a small vase near the head of each child. Bottom body of the mother at the death scene. Note the advanced putrefactive changes and the rose placed inside a small vase near her head

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Fig. 2
figure 2

Body of the father at the death scene. The plastic bag and loose rope are clearly visible. Note the lack of putrefactive changes when compared to the bodies of the mother and two sons. The empty CO2 container can be seen slightly obscured by the bed in the upper left corner of the image

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His head appeared cyanotic and the conjunctivae of the eyes showed evidence of occasional petechiae. The rest of the body did not reveal any signs of a violent struggle nor did it show signs of putrefaction. Postmortem lividity (livor mortis) was strongly developed and disappeared briefly when pressed. A 500 g single-use CO2 cylinder was standing on the floor (Fig. 3). A “Sofortsektion” (immediate autopsy) of all four bodies was ordered by the state prosecutor.

Fig. 3
figure 3

One of the two CO2 containers discovered inside the apartment. This container was empty and discovered near the body of the father (see Fig. 2). The second empty bottle was discovered in the bathroom. Note the label which describes the contents and use (i.e., 500 g, CO2 aquarium plant fertilizer)

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Autopsy report

Atherosclerosis of the coronary arteries and a recent infarct of the left ventricular anterolateral papillary muscle were the only remarkable findings during the autopsy of the husband.

The autopsies of the mother and two sons showed remarkably advanced putrefaction in a short space of time. No further morphological changes were observed during the autopsies or following postmortem computed tomography (PMCT). Femoral vein blood samples were submitted for chemical–toxicological analyses and showed minimal levels of alcohol in the samples from the husband (0.3 ‰). Toxicological analyses of the mother and sons did not show the presence of any toxic substances.

Further investigations

Further investigations conducted by the “Landeskriminalamt” (LKA, State Office of Criminal Investigations) revealed the presence of a second 500 g single-use CO2 cylinder in the bathroom. This bottle and the one found near the body of the husband were almost empty.

According to the labels, the containers were sold as CO2-fertilizer for aquarium plants. However, the apartment did not contain an aquarium. On further inspection two synthetic inhalation face masks and tubing were found on top of a chest of drawers (Fig. 4). Analysis for DNA revealed a mixture of all four decedents on the masks. There was only a minor amount of the husband’s profile discovered, with a single source trace of the youngest son’s profile.

Fig. 4
figure 4

Two inhalation face masks discovered on a table in the apartment. Mixed DNA traces of all four family members were recovered from the masks

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Cause of death

The cause of death in all four cases was CO2 toxicity, after taking into consideration the scene findings, the reconstructed sequence of events, the autopsy, and outcome of toxicology.

Discussion

Many CO2 accidents involve fermentation tanks, brewery and wine cellars, waste water tanks, sewage tanks, and other types of storage tanks, due to the release of CO2 during the fermentation process and the resulting accumulation of high concentrations of CO2 at the bottom of these contained spaces [5–8].

Furthermore, accidental and suicidal deaths in connection with dry ice have also been reported [1, 9–12]. Dry ice is the solid phase of carbon dioxide, which changes at −78.5 °C, through sublimation at atmospheric pressure, into its gaseous state.

At 20 °C and a pressure of 55 bar CO2 changes into its liquid state and can be stored in gas bottles [1, 13]. It is possible to determine the reduction of O2 and the increase of CO2 in the air inside a room/space by measuring the atmosphere at the scene or by conducting experimental measurements [1, 7–9, 12]. The “normal” atmospheric air contains a minimum CO2 of 0.03–0.04 %, an O2 content of approximately 21 % and an N2 content of 78 %. The physiological partial pressure of CO2 (pCO2) inside the human body generally measures 35–45 mmHg, which roughly equals 4.5–5.8 vol% at normal pressure. Individuals with chronically increased pCO2 levels may tolerate higher pCO2 levels in their environment.

A pCO2 of approximately 60–70 mmHg (7.7–9 vol% at normal pressure) is considered a pathological concentration. Carbon dioxide stimulates the circulation, respiration, and heart rate at slightly elevated pCO2 values (<10 vol%); i.e., blood pressure increases, tachypnea, dyspnea, tachycardia, and tachyarrhythmia commence. Neurological changes can be observed at levels above 10 vol%; restlessness, headaches, and anxiety are commonly reported, followed by numbing effects such as clouded awareness, reduced excitability, and increased paralysis. Carbon dioxide levels above 10–20 vol% pCO2 result in seizures, coma, and finally death [2–4].

The cerebral circulation has a limited self-protection mechanism, the autoregulation mechanism It acts by increasing cerebral blood circulation during hypoxia and hypercapnia through direct myogenic effects without nervous stimulation, while it decreases the circulation during hypocapnia [14]. Circulation is increased to ensure that enough oxygen reaches the brain in cases of low oxygen and high CO2 blood concentrations; however the mechanism fails when pCO2 levels are too high. The inhalation of a gas mixture high in CO2 concentration leads to a lowering of the intracellular O2 concentration within a few seconds. This is called hyperacute anoxia, which is also commonly found in hanging [15, 16]. Severe anoxia leads to a breakdown of cognitive functions, unconsciousness, and after a few minutes, irreversible brain damage [3].

The mechanism of CO2 intoxication in the cases described in this paper is not fully known, and had to be reconstructed from the findings at the scene and during background research conducted by investigating officers. An example of the effective use of 100 % concentrations of CO2 is the euthanization of laboratory animals, where quick-acting and painless methods with minimal stress are required [17]. Other quick-acting and painless methods of suicide using anesthetic/narcotic agents have been reported previously and are almost exclusively linked to the medical professions [18].

It is hypothesized that the husband caught his wife and children off guard while they were sleeping, slipping the inhalation mask over the mouth and nose of each sleeping individual in turn. The inhaled pure CO2 would ensure unconsciousness within a few seconds due to hypercapnia and severe anoxia. The rapid increase of CO2 concentrations in the brain would render the victim helpless, probably leaving the victim no time to wake and defend themselves, or others.

Within a few minutes irreversible brain damage would have affected the brain stem and in particular the respiratory and circulatory centers, leading to respiratory arrest. It is also possible that significant cardiac arrhythmia and/or respiratory acidosis with severe clinical symptoms may have accelerated the fatal outcomes.

The circumstances of the father’s death seemed puzzling considering the presence of occasional conjunctival petechiae. It is hypothesized that he filled the bag with CO2 remnants from the container and then pulled the bag over his head, tightening it with the rope. There may not have been enough CO2 left in the container and he may have suffocated by his own exhaled CO2., in addition to the effects of pressure on the neck. This scenario could explain the petechiae.

Carbon dioxide is virtually impossible to detect during autopsy or by toxicological analysis. Kettner et al. [5] reported on the rapid and elaborate extraction of CO2 content in the lungs of the victim using QMS 421 C (quadrupole mass spectrometer) in an accidental CO2-related death inside a fermentation tank. The analysis demonstrated an increased CO2 concentration and decreased O2 concentration; however, the diagnostic value decreases with increased body decomposition [5]. Opinions on the diagnostic value of postmortem CO2 analysis vary. The scientific literature reports that postmortem CO2 concentrations are always raised and can therefore not be interpreted correctly [1].

The role of the forensic pathologist in the aftermath of a mass disaster with high levels of atmospheric CO2 has been well documented by Wagner et al. [19]. The case described by Wagner et al. illustrates how whole villages succumbed to a gaseous cloud of CO2 released from a lake. The positions of the victims suggested that they experienced a rapid loss of consciousness and died carrying out their daily routines.

Key Points

  1. 1.

    Carbon dioxide is vital for living organisms but highly toxic in raised concentrations. This paper reports the use of CO2 as a potent weapon in a murder-suicide case, where the death scene offered virtually no clues as to a lethal modality and the autopsies were nonspecific.

  2. 2.

    Evidence of CO2 intoxication during autopsy and toxicological analysis is almost impossible to obtain if such intoxication is not suspected and the body is in an advanced state of decomposition.

  3. 3.

    Pure CO2 in large quantities is relatively inexpensive and freely available to the general population.

  4. 4.

    The forensic pathologist should consider CO2 poisoning as a possible cause of death if the cause of death is unclear. Information gathered from the death scene during the investigative process is crucial in providing indicators of possible CO2 poisoning.