The Smell of the Cinema: Human Chemical Signals?

By Neuroskeptic | May 12, 2016 4:01 pm

The air in a cinema contains a chemical cocktail emitted by the audience – and the emotional tone of the movie influences the molecular composition of the cloud.

That’s according to a striking set of results from researchers Johnathan Williams and colleagues who took air samples from two 230-seater screens of a cinema in Germany over a period of two weeks.


Here’s an example of the chemical trace associated with shows of the movie “The Hunger Games 2: Catching Fire”, featuring three chemicals – carbon dioxide (CO2), acetone and isoprene. It’s obvious that levels of all three increase over the course of the movie, as they accumulate in the air from all the people in the audience:

hunger_gamesBut more interestingly, we also see two peaks in the level of CO2 and isoprene towards the end of the movie. These peaks turn out to correspond to two especially tense moments in the film. This association makes sense because emotional stress increases heart and breathing rate, which will tend to increase the amount of CO2 exhaled. As for the isoprene, this is believed to be released by muscle activity – perhaps the isoprene peaks reflects people moving or ‘fidgeting’ in tension.

Williams et al. report that there many more associations between all kinds of molecules and multiple different emotional states; they go on to speculate that these chemical secretions might be important (although it’s unclear whether they are detectable by human noses)

Humans possess a very well developed sense of smell, and new evidence suggests that recall is more effective, and our perception of faces changes with odours present. Therefore the chemical accompaniment generated by the audience has the potential to alter the viewer’s perception of a film… These findings also have obvious industrial applications where an objective assessment of audiovisual material is sought from groups of people, for example, in advertising, video game design or in film making.

However, do the stats hold up? I’m not sure.

Here’s how Williams et al. describe their data analysis approach:

Data mining algorithms were applied to analyze the [chemical composition of the air] and label data within a 10 minute window around a given measurement datapoint (5 minutes backwards and 5 minutes forwards). The first method applied was forward prediction, whereby the [chemical concentrations] are predicted based on regression from past [concentrations] and the film labels. The second method was termed backward prediction, as it used [chemical] changes ahead of a given point in time to predict the current associated label.

However, one problem with this approach is multiple comparisons: Williams et al. examined lots of different chemicals (over 100), and dozens of scene content labels, so there must have been thousands of potential associations tested. The authors rather dubiously justify their decision not to apply any multiple comparisons correction as follows:

Whereas in most cases, an adjustment like Holm-Bonferoni should be performed on the tests, this is not necessary in this case, as we only searched for indications for further analysis, which we also can get from uncorrected values.

ResearchBlogging.orgWilliams J, Stönner C, Wicker J, Krauter N, Derstroff B, Bourtsoukidis E, Klüpfel T, & Kramer S (2016). Cinema audiences reproducibly vary the chemical composition of air during films, by broadcasting scene specific emissions on breath. Scientific Reports, 6 PMID: 27160439

CATEGORIZED UNDER: movies, papers, select, Top Posts
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  • Uncle Al

    April 2016 407.57 ppm CO2 in air, Mauna Loa Observatory. German cinema, up to 1600 ppm CO2. Conclusion: Ban theatres – reverse Klimate Kaos, sea level rise, and the Carbon Tax on Everything. Klimate Kaos is a thespian conspiracy!

    Do gyms have enormous airborne isoprene concentrations?

    • zlop polz

      “German cinema, up to 1600 ppm CO2”
      Is the lapse rate different in German cinemas?

  • zlop polz

    How does that correlate with popcorn/hot-dog consumption ratio?

    • Neuroskeptic

      A very good question. When I said that “The air in a cinema contains a chemical cocktail emitted by the audience” this is true in a broad sense – the audience, including anything they’re carrying, emits the chemicals.

      We certainly can’t assume that all the emissions are via breathing.

      • zlop polz

        Perhaps, United Nations IPCC scientists need to determine climate perturbations of
        infrared blocking chemicals, escaping from theaters. Do these chemicals lower the lapse?

  • Joanne Williams

    How were the different showings handled? Seems like they could have built their regression models on a training set and then cross-validated it on testing showings. Multiple comparison wouldn’t be a problem then.

  • Uncle Al

    Isoprene is an EPA priority carcinogen, PMID:12571689. Prohibit all movie theaters.

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  • Amos Zeeberg

    Shouldn’t there be some latency between when the reputed emotional trigger comes and then the observed difference in air chemistry? It takes a while for the metabolic changes to produce the different chemicals and then takes a while for the exhaled chemicals to mix through the theater and then get pulled out through the vents in the ceiling. I don’t see that they accounted for that delay.

    • Neuroskeptic

      That’s a good point. If you look at the graphs, it does look like there’s an onset and offset to the chemical “peaks”.

  • Tiffany Summers

    Is this suggesting that humans give off pheromones in
    the same way as other animals, but humans just lack the necessary olfactory capabilities to detect them due to the regression of the vomeronasal organ? It doesn’t make sense for an organism to do
    something like this when it has no use. Is it possible that as humans evolved, these chemical signals were no longer necessary? Are pheromones just a residual feature similar to the appendix in humans?

    • Neuroskeptic

      That’s an interesting question, but the molecules examined in this study weren’t pheremones, and most of them probably have no signalling function.

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Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.


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