Cushion make-up is essentially a liquid foundation that is immersed in a sponge and applied with an applicator pad. The user presses the applicator pad onto a sponge that is soaked with liquid foundation, and then pads or rubs the applicator pad over the skin to dispense the product.
I am not a big fan of applicator assisted application of liquid foundation including cushion foundations, as both the applicator pad and the cushion may accumulate dirt, mould and bacteria over time. When the applicator is moved back and forth over the skin, it will pick up bacteria from the skin surface, which are then transferred onto the cushion and into the liquid foundation reservoir underneath. As these products are also promoted for repeated touch-ups during the day, this process might take place several times per day, which makes the accumulation of bacteria even more likely.
The warmer and more moist, the higher the likelihood of bacterial overgrowth (bacteria love wet, warm, dark environments!). Also the more air contact, the higher the risk of turning your product into a breeding ground for microbes. The sponge sitting in the liquid foundation reservoir is highly porous (the sponge may contain up to 800,000 individual pores), which increases the contact of the liquid with air. This combined with the mechanical action of repeatedly stamping the applicator pad onto the porous sponge ‘pumps’ even more air into the liquid.
The liquid foundation will of course contain some form of preservative (as most skincare products do) to prevent bacterial overgrowth, and the sponge and applicator are sometimes claimed to be anti-bacterial also. However, the technique of repeatedly rubbing an applicator over the skin and then back over the cushion is in my opinion unhygienic and I wouldn’t recommend it.
I could imagine that cushion foundations require a higher concentration of preservatives to keep them safe, which might in itself be a potential problem, as preservatives have the potential to cause skin irritation.
In my opinion application of liquid foundation with freshly washed hands is the most hygienic way to apply it. To simply squeeze liquid foundation out of a tube or bottle and apply it with clean hands has a lower risk of breeding bacteria in the container, compared to cushion foundation.