Sinister Synergists

By Joe Charbonnet | October 1, 2019

Synergist chemicals quietly keep a low profile, but they are a crucial part of flame retardants’ act: Think of them as Teller to flame retardants’ Penn. 

These little-known compounds don’t slow ignition on their own, but when synergists are mixed with  flame retardants, they can help delay fire spread. Synergists help create less-flammable chars or quench chemical reactions during burning. And we use a lot of them: While flame retardants can make up 15% of the weight of some plastics in electronics cases, synergists can comprise another 5-10%.

Because they aren’t technically flame retardants, our exposure to synergists from products and dust receives little notice. But we ought to be paying attention, because synergist chemicals like antimony trioxide (Sb2O3) can be toxic. Antimony trioxide likely causes cancer in humans. Long-term exposure can damage the lungs, kidneys, liver, and heart. This chemical may also harm developing fetuses and the male reproductive system.

Compounds of zinc bound with tin or molybdenum atoms (known as zinc stannates and zinc molybdates) are also sometimes used as synergists. These chemicals are poorly studied for their long-term toxic effects and may also harm health. Certain tin-containing chemicals, for instance, contribute to obesity risk.

Fortunately, regulators are starting to pick up on the dangers of these chemicals. A recent bill in Massachusetts to regulate flame retardants also bans antimony trioxide in many applications. Both New Jersey and California have designated the compound a cancer-causing hazard.

Industry and academia have also started to research alternatives, including synergists based on clay and carbon nanotubes. While these compounds have not yet been proven safe to humans, this work reflects a growing awareness that our current exposure to antimony trioxide should be reduced.

When future flame retardant regulations are put into effect, they should also consider the synergists found in many of our products. Just because the synergists are in a supporting role doesn’t mean our health should play second fiddle.