The terrarium of a chameleon should have proper ventilation. If the air inside the terrarium is not refreshed often enough, your chameleon can get upper respiratory infections or get ill in general. Mold and stale are will fester inside the terrarium and will make it a bad thing to have in your house and unhealthy for you chameleon to live in. It is also much harder to regulate the air humidity in a terrarium without proper ventilation.
A terrarium that is sold as housing for snakes and lizards generally does not have enough ventilation for a chameleon. Just one slit with mesh or one area with mesh is not enough. A terrarium for a chameleon needs at least two sides with a big portion of mesh. If you already have a terrarium without too much air circulation, you can replace a door for a mesh door or can install mechanical ventilation like a fan. If you are handy you can install an electric computer fan on one of the ventilation slits of the cage. You can have it suck air out of the terrarium. On the other end this will create an inflow of air, allowing it to be replaced without creating too much draft. These computer fans are cheap and very silent, making them excellent to use in your terrarium.
Net cages for chameleons
A net cage is an enclosure for reptiles that consists entirely of netting or mesh. It is also called a flexarium or screen terrarium. It is often fold-able and lightweight. It will provide the maximum amount of ventilation for a chameleon, but this could be too much. A terrarium should stay a bit humid, and in a net cage this is very hard to maintain. Usually outside and inside the net cage the air humidity and temperature will equalize. If you live in a humid climate this is fine, but in most cases this results in a too low level of air humidity.
I would advice to only use net cages temporarily, for example when you take your chameleon outside for a bit. You can place the chameleon in the cage and hang it somewhere safe and half in the sun, half in the shade. Your chameleon can benefit from the light and heat of the sun but can also hide from the heat in the shadow.