The answer to the question, "If the nose knows, or does it?", is mostly false.

The majority of people have a sense of smell that is acute and sensitive enough to detect odors in the parts per million (ppm). Ozone, for example, can be detected at concentrations as low as 0.01 ppm. However, it is not dangerous until the 8-hour time-weighted average (TWA) for the permissible exposure limit (PEL) of 0.1 ppm is exceeded.

On the other hand, isocyanates, which are used in many single and two-part polyurethanes have poor olfactory warning properties. This means that if a person detects its odor, he has already exceeded the OSHA1 or ACGIH2 exposure limits. For example, one form of isocyanate, TDI's (toluene 2, 4-diisocyanate) has a PEL of 0.02 ppm over an 8-hour TWA, and due to the chemical's poor warning properties, a person's exposure would exceed this limit if an odor is detected.

In some situations when using a solvent-based adhesive, such as one that contains methylene chloride, employers should establish safe use procedures that incorporate engineering controls and the use of personal protective equipment to minimize exposure. Even better would be to replace the solvent-based product with a solvent-free adhesive. In the case of most solvent-containing materials, the nose will provide ample warning to an employee that an odor/exposure is detected, but it will not tell them if it's harmful or if exposure limits have been exceeded.

The best way to learn more about a particular chemical or adhesive is to consult the material safety data sheet (MSDS). The nose should never be used to determine if a product is harmful. Learn more about Dymax global certifications and compliance.

Why does this new adhesive smell so much more than our current product?

The short answer is that a person's sense of smell, through prolonged exposure, becomes desensitized to the current odor. When a different odor is detected your nose smells only the new odor.

Additionally, people develop olfactory fatigue which is a temporary inability to detect a particular odor. For example, when you enter a restaurant and smell freshly cooked garlic, this odor is only noticed for a short period. When a new odor is introduced, let's say, freshly baked cookies, your olfactory senses detect the new aroma but not the garlic that smelled earlier.

After prolonged exposure to a new adhesive, an employee will also become desensitized to this material and if you were to switch back to the old adhesive this one would smell a lot worse.

Does the new adhesive smell worse? Maybe, but it is more likely that it's just different and your nose has yet to become accustomed to it.