You’ve smelt it – that weird smell on your hands after touching coins or the smell of period blood on your hands. It’s so specific and, well, metallic.
There’s a misconception that when we smell blood we are smelling iron – metal. This isn’t quite true. It’s weirder than that. Your period is blood, and so when we talk about why your period smells metallic, we’re really talking about why blood smells metallic.
There are two ways that (period) blood can smell metallic. The first way is when the iron in blood comes into contact with our skin (the vulva or our hands). The second way is when the iron in blood interacts with the same molecules, but inside blood, making blood on a tampon – that hasn’t touched our oily skin – still possibly smell metallic.
Period blood on a pad, on our vulva or on our hands will have a stronger metallic smell than blood on a tampon, particularly if we are oily or sweaty.
So here goes…
The metallic smell of blood is a form of body odour. It’s not the iron that smells; it’s the iron interacting with the oils on your skin (or with the same components from the skin that are also in blood). The ‘smell of blood’ is the smell we make when iron comes into contact with oils on our skin.
The oils on our skin break down when they come into contact with metals. In fact, a whole new molecule is created during this interaction, and that’s the one that we perceive as smelling metallic.
The smell of metal on the skin is not the smell of metal, but the smell of metal interacting with our skin (or the blood with itself).
The science stuff
When lipid peroxides (skin oils) in sweaty skin decompose (corrode) iron (in blood or metal), they produce an odorous carbonyl hydrocarbon mixture containing volatile compounds (hexanal, heptanal, octanal, nonanal, decanal and 1-octen-3-one).
During the decomposition interaction, the iron molecule gains two electrons, resulting in Fe2+ ions. The iron becomes twice as negative, reacting with the oils in our skin, with the decomposition quickly resulting in Fe3+ ions and the carbonyl hydrocarbons.
What this means in plain English
These smelly compounds are what result in the perception of a metallic smell. This smell is a sensory illusion – there are many items in our world that have these compounds. Some we deem to smell ‘like blood’ or ‘like metal’, but according to scientific research, this association is likely to be as much to do with visual clues as anything.
A good example of this is that the smell of blood doesn’t make people faint; the sight of blood does. These visual cues, such as the colour red or seeing blood or touching a metal tool, create an association with the smell of either metal or blood, depending on the situation.
When study participants were asked to identify this smell (created artificially and they didn’t know what it was supposed to be) they identified it as a huge variety of things including cleaning mixture, hospitals, metallic, mushrooms, forest, and stagnant water. There were almost no themes, and certainly, it wasn’t universally identified as blood.
The same odourant production process occurs with copper and brass. A sweaty hand on a copper doorknob makes the doorknob and your hand both smell. The carbonyl hydrocarbons are transported around by your greasy mitts.
The odour of jewellery, particularly older jewellery, is another good example because the jewellery touches our skin and its scent can change. The oilier your skin, the stronger the odour.
Our blood can also contain these fats (lipid peroxides), meaning that iron in the blood can react with the fats in the blood, causing blood to smell metallic without it coming into contact with our skin.
Interesting perspectives on the smell of blood
The smell of blood: fear or food?
An experiment was conducted to look at the perceptions and emotional responses of men and women exposed to the artificially-simulated smell of fresh blood in contact with the skin.
The authors of the study figured that the smell of blood – and its associations with injury, danger, death and food – would be a cue to activate fundamental motivational systems relating to either predatory behaviour or a prey-like withdrawal behaviour, or both.
That is, the researchers wanted to find out what we’d do when we smelled blood – run away in fear or get our napkins ready.
The results showed that we’re all strongly affected by the smell of blood in both positive and negative ways and that women are more sensitive. Women have been found to be more sensitive to smells than men across many studies, but whether this is biological or learnt isn’t clear.
Women and men’s emotional responses to the smell of blood could be divided into strong positive and negative ratings, with negative ratings in women having a strong arousal component. (Arousal doesn’t always mean sexual – when we are excited, scared or anxious, we are also aroused.)
The split in women’s positive or negative responses was related directly to the phase of the menstrual cycle they were in at the time and if they were on the oral contraceptive pill (the pill).
Key findings of the study
- Study participants either liked the smell of blood or they did not
- For women, whether they liked or disliked the smell was related to whether they were in their fertile phase or on the pill
- Women who were close to their fertile phase (ovulation) had a more positive rating to the blood smell
- Women are known to be more sensitive to odours during various phases of their menstrual cycles overall, even women who are on the pill
- Women on the pill found the simulated smell of blood more arousing than men, but only women who had a negative response
- For this reason, women may be more sensitive to withdrawal cues (the urge to run away), but according to this study, it seems to be only women who are on birth control
- Women who were not on the pill rated the smell of blood more positively, and those who were in their fertile menstrual phase even more so
- Men display a similar split in their ratings of the smell of blood
- Vegetarians and vegans had significantly lower arousal, either positive or negative
- This study, like any good study, creates way more questions than it answers!
References
Angew Chem Int Ed Engl. 2006 Oct 27;45(42):7006-9.
The two odors of iron when touched or pickled: (skin) carbonyl compounds and organophosphines. Glindemann D1, Dietrich A, Staerk HJ, Kuschk P.
Moran JK, Dietrich DR, Elbert T, Pause BM, Kübler L, Weierstall R. The Scent of Blood: A Driver of Human Behavior?. PLoS One. 2015;10(9):e0137777. Published 2015 Sep 23. doi:10.1371/journal.pone.0137777
Bertomeu-Sánchez Chemistry, microscopy and smell: bloodstains and nineteenth-century legal medicine. JR Ann Sci. 2015;72(4):490-516. doi: 10.1080/00033790.2014.974069. Epub 2015 Feb 3.
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