One of the questions we are most frequently asked is how much “odor” our aid corresponds to: how much odor it actually generates and whether that odor can be compared to the one produced by the same substance in a different quantity. Although we would like to provide a clear and straightforward answer, the reality is that we cannot, because the problem is deeply complex.

We are used to thinking that if there is more of something, it should smell more. However, when it comes to odor, things are not so simple. There is not necessarily a direct or linear relationship between the total quantity of a substance and the odor perceived. The key idea to understand from the beginning is this: odor does not depend on how much solid material is present, but on how many molecules enter the air and how they interact with the environment.

To better understand this, it is important to start with the physicochemical properties of substances. These are intrinsic properties of each molecule: a molecule is what it is, and therefore behaves in a specific way. Some molecules are volatile and easily transition into the vapor phase (green solid example, Image 1); others are not (yellow solid example, Image 1), but degrade or transform into volatile compounds that we do perceive as odor; and there are substances that do not volatilize at all and are therefore considered to be “odorless” (orange solid example, Image 1). These properties determine which molecules reach the air, how many do so, and at what rate, forming the basis of each substance’s characteristic odor.

In addition to the intrinsic properties of the substance, the role of the environment must also be considered. Temperature and pressure directly influence the amount of odor released. Generally, higher temperatures lead to greater volatilization and stronger initial odor perception, but also to a shorter duration over time, as shown in the red thermometer example (Image 2). A common everyday example is perfume: it lasts longer in winter, while in summer its scent is more intense but fades more quickly.

Humidity, airflow, and the volume of the space where the substance is located are equally decisive. Odor does not behave the same way in a closed space as it does outdoors, nor when a substance is contained inside a box in a humid garage compared to being fully exposed to the air. As a result, the number of molecules in the vapor phase can vary greatly even for the same amount of solid material. Therefore, large quantities of a substance may generate the same odor concentration as much smaller quantities under different conditions. Odor is not something that simply “adds up,” as weight does: more substance does not necessarily mean more odor.

Calculating odor concentration in air is extremely complex, as it requires considering all these variables simultaneously. But even if all external variables could be controlled, one key factor would remain: understanding how the “instrument” that perceives odor works.

In the case of dogs, that instrument is their olfactory system. Depending on the physicochemical properties of the molecules and their affinity for olfactory receptors, the response can vary greatly. In some cases, very low concentrations are sufficient to generate a strong response, even saturating the receptors. In others, much higher quantities are required for the odor to become perceptible.

This is important because when a receptor becomes saturated, adding more odor does not produce a stronger response. Beyond that point, the olfactory system perceives “one” and “seven” as the same signal. In other words, the instrument can no longer distinguish how much more is present. This lack of proportionality is not exclusive to biological olfaction; it also occurs in artificial detection systems such as electronic noses. Due to their high sensitivity, these systems can easily saturate at high concentrations and produce the same signal for very different quantities of a substance. For example, in our gas chromatography–mass spectrometry system, a 1 kg sample of TATP may generate a signal similar to that of a 100 g sample, without implying that the actual amount of volatile compounds in the environment is the same.

A simple example helps illustrate this. Imagine entering a kitchen where someone has eaten mandarins (Image 3). Would you be able to tell how many? Not whether there were two or three, but simply whether there was one or more than one. Mandarins emit a strong odor and our receptors saturate easily. In a closed space under stable conditions, there comes a point where, even if more substance is present and more molecules are released into the air, the olfactory response remains the same.

The combination of molecular properties, environmental conditions, and chemical processes such as phase equilibria — which explain why, under certain conditions, a large amount of solid can generate the same airborne molecular concentration as a much smaller quantity — means that a direct equivalence between “amount of substance” and “odor generated” cannot be established. This relationship is not necessarily linear because it depends on multiple external factors, and its precise quantification is also limited by the capabilities of measurement instruments.

Therefore, any numerical value provided would only be valid under very specific conditions and would cease to be accurate as soon as those conditions changed.