The odor associated with menstruation is a common experience. This scent often becomes stronger the longer the fluid is exposed to air and remains in a sanitary product. Understanding why this happens provides a straightforward explanation for the metallic or sometimes musty scent that many people recognize. The changing odor of menstrual fluid is a natural consequence of its complex makeup and the chemical reactions that occur outside the body.
The Composition of Menstrual Fluid
Menstrual fluid is not solely composed of blood, which influences its eventual odor profile. The discharge is a complex biological mixture, typically about half blood and half other components. This fluid includes shed endometrial tissue, mucus from the cervix, and various vaginal secretions.
The blood component contains iron. The other components, particularly the tissue and secretions, are rich in proteins and organic matter. This combination provides the raw materials for the natural chemical and biological breakdown processes that lead to the fluid’s characteristic scent.
The Science of the Odor: Oxidation and Bacterial Action
When menstrual fluid exits the body and is exposed to the environment, two primary mechanisms begin to change its smell. The first is oxidation, which creates the recognizable metallic scent. This smell, often compared to copper pennies, is caused by the iron within the hemoglobin molecules reacting with oxygen in the air.
This chemical change occurs quickly and is the source of the slight metallic aroma often noticed shortly after a product is changed. The second and more prominent source of strong odor is the action of bacteria. Menstrual fluid is a nutrient-rich medium for the naturally-occurring bacteria that reside in the vagina and on the surrounding skin.
When the fluid is contained within a pad, tampon, or cup, it creates a warm, moist environment where these microbes proliferate. As these bacteria metabolize the proteins and tissue in the fluid, they produce various metabolic byproducts. These byproducts are released as volatile organic compounds (VOCs), which are gases detected as strong or musty smells.
The longer the fluid remains in the sanitary product, the more time the bacteria have to break down the organic matter. This prolonged decomposition process transforms the initial metallic smell into the stronger, musky, or earthy odor associated with “old” blood.
When Odor Signals a Health Concern
While a slight metallic or musty scent is normal, certain distinct odors can signal an underlying health issue. A strong, intensely fishy odor is often a sign of a bacterial imbalance, such as Bacterial Vaginosis (BV). This characteristic fishy smell is caused by the production of amines by an overgrowth of certain bacteria.
This odor may become more pronounced during menstruation because blood slightly raises the vaginal pH, creating a less acidic environment that favors the BV-associated bacteria. A sweet or bread-like odor may indicate a yeast infection, which results from an overgrowth of yeast. Yeast infections do not usually have a strong, unpleasant smell but are often accompanied by thick, white discharge and itching.
A very foul or rotten smell, distinctly different from the normal musky scent, should prompt immediate attention. This odor is often the result of a retained foreign object, such as a tampon left inside the vagina too long. Any highly offensive odor, particularly if accompanied by fever, pain, or unusual discharge, warrants a consultation with a healthcare provider.
Practical Steps for Odor Management
The most effective strategy for managing normal menstrual odor is to minimize the time available for oxidation and bacterial action. This is achieved by changing sanitary products frequently, regardless of flow. For pads and tampons, this means changing every few hours, and for cups, emptying and rinsing at least twice daily.
Choosing highly breathable undergarments, such as cotton, can also help by reducing the moisture and heat that encourage bacterial growth in the external area. When practicing hygiene, it is important to only cleanse the external genital area (the vulva) using mild, unscented soap and warm water. Internal washing or douching should be avoided, as this disrupts the vagina’s natural, self-cleaning bacterial balance, which can increase the risk of abnormal odors or infection.