FAA & Toiletry Company Conspiracy on Volume vs Weight?
Consider the example of a bottle of Paul Mitchell Spray Wax (hairspray). This lightweight aerosol has the following volume and weight specifications: 125ml, 89g / net weight 2.8oz.
At 2.8oz, this is clearly under the 3.4oz weight limit and clears TSA screening easily at any U.S. airport, at only 82% of the limit. However, I've had it confiscated at Heathrow and Toronto because they focus on the volume being 125ml, which is 25% over the 100ml limit.
When the liquid is water, there is no problem, because 1ml (volume) = 1g (weight) = 0.0353oz (weight). So 3.4oz of water = 96.4g = 96.4ml and all tests are passed.
But not all liquids have the same density and different volume are required to accommodate the same weight across products, and vice versa.
U.S. manufacturers determine product sizes for toiletries in part based on carry on rules. The FAA needs to switch to the volume rule (or get the rest of the world to use the weight rule), so that toiletry manufacturers can make a size that will clear security for both directions on an international trip.
Then again, maybe this is a toiletry industry conspiracy happy to see our $20 hairspray taken so we are forced to buy more prematurely.