I work next to an open window, but I wear a standard dust mask available at any do it yourself shop, usually can also find the better versions with changeable filters and have seal around the nose and mouth, if you can't find this type at the do D.I.Y. shop you will certainly find them at trade centres, Dulux or Crown trade centres for example, I would suggest you done buy these at airbrush shops because you will pay more than double the cost.
You will be safer with the better mask if your planning on 2K clearing.
I forgot to mention, I over thin and work at very low pressure, usually between 5 and 10 PSI, if you are spraying at higher pressures you may need an even better mask.
Water based paints are less harmful than uro's but they still contain harmful substances.