Basics

A liquid that can easily turn into a gas at or near room temperature
Is used to drastically increase the capacity of gas propelants in things like aerosol cans since more can be stored as a liquid than a equivalent final volume of compressed air (or most other gasses for that matter)
Can be pretty nasty chemicals (like Chlorofluorocarbons (CFCs) )
Many are also quite difficult to produce due to the use of Florine Containing Compounds, such as in the banned Chlorofluorocarbons (CFCs) (Which were used in pretty much every aerosol product until it was discovered they disrupted the ozone cycle in the upper atmosphere and they were subsequently banned), or in the still used hydrofluoroalkanes (HFA) (In Medical Inhalers)
Many non-florine based propellants are fossil fuel based (Propane,Butane,etc)
Lukily there are some that are effective, safe, and renewable
This Page Explores Those Propellants

Possible Propellants

DME or Dimethyl Ether
You may have heard about it due to it's sutability as a fuel.
Currently used in % of the total aerosol products on the market
May be usable as an alternative to florine compounds in medical inhalers. Needs more research + expert opinion before we pursue it for this appliciation, but a quick google search brough up This study in which a propane DME mixture was used for an expiremental medical inhaler.
Has the one issue of flammability

N2O or Nitrous Oxide
Harder to Produce
Has the advantage of being foodsafe
Also is less soluable than CO2 in water thus making it more suitable for food applications such as whipped cream (It also doesn't produce carbonic acid which curdles the cream)
Has the potential for abuse as a recreational drug

There are three main areas of health concern linked to aerosol cans:

Aerosol contents may be deliberately inhaled to achieve intoxication from the propellant (known as inhalant abuse or "huffing"). Calling them "canned air" or "cans of compressed air" could mislead the ignorant to think they are harmless. In fact, death has resulted from such misuse.[18]
Aerosol burn injuries can be caused by the spraying of aerosol directly onto the skin, in a practice sometimes called "frosting".[19] Adiabatic expansion causes the aerosol contents to cool rapidly on exiting the can.
The propellants in aerosol cans are typically combinations of ignitable gases and have been known to cause fires and explosions.[20]

In the United States, non-empty aerosol cans are considered hazardous waste.[20]