What's the difference between TDI, MDI, and HDI isocyanates?

TDI, MDI, and HDI are all types of isocyanates, which are chemicals commonly used in manufacturing, especially in making foams, coatings, and adhesives. The main difference between them lies in their structure and how they're used.

TDI (Toluene Diisocyanate) is mostly found in flexible foams, like in furniture or bedding. It’s fairly volatile, meaning it can easily become airborne, which makes inhalation a key concern in workplace exposure.

MDI (Methylene Diphenyl Diisocyanate) is more commonly used in rigid foam insulation and some adhesives. It’s less volatile than TDI, so it’s not as easily airborne, but exposure is still a risk, especially when sprayed.

HDI (Hexamethylene Diisocyanate) is often found in spray paints and industrial coatings. It’s used in automotive paints and finishes. HDI is typically found in polyisocyanate blends, which can be particularly hazardous in spray applications.

Even though they differ in volatility and application, all three can cause respiratory sensitisation — meaning once a person is sensitised, even very low levels can trigger asthma-like reactions.