Orchids do not like stagnant air. In their natural habitat, most orchids grow high up in trees where the breezes are always blowing.

Wind cools the leaves when it is hot, and helps dry excess moisture that may have accumulated on the plant. Wind also helps distribute warm and cold air so harmful air pockets don't form.

Because most greenhouses are very airtight, the need for mechanical ventilation is a must.
Many greenhouses have roof and/or side vents that can be opened to vent hot air out and let cooler air in.
Some of the side vents may also be equipped with electric fans.

When the temperature reaches a preset level, the fans turn on and exhaust hot air to the outside.
This not only helps cool the greenhouse but also replaces stagnant air with fresh air.

You may also install a simple duct system consisting of a large plastic tube that has vent holes cut into it every two or three feet. A small blower at one end of the tube blows air through the tube and out the vent holes.

This keeps the air moving in the greenhouse. If the roof of your greenhouse is tall enough, ceiling fans also work very well.

In your home, the easiest way to create air movement is to open a window.
Be careful of drafts because they can be harmful. A draft of cold air can cause buds to drop.

If a window can't be opened, use small fans directed away from your orchids to circulate the air in the room (ceiling fans work great).
The photo shows what air pollution does to orchid blooms, in this case a Cattelya.

In December of 2000, there was a 4-day period of alert days of extreme pollution caused by a thermal inversion in the Salt Lake valley.

The air quality inside my greenhouse became so poor that many of my blooming orchids were effected like this one.

The one on the left never opened any further than this and the petals just dried up. The one on the right followed the fate of the first flower a day later.

In the home, accumulated air pollutants from pilot lights, smoking, cooking, aerosol sprays, plastic and other synthetic materials, people, and other sources can all be harmful to your plants.

To reduce the amount of pollution in your home, open a window to let fresh air in (of course, you don't want to open your widows if it is extremely smoggy outside because you may be letting more pollutants into your home than you are trying to remove).

Electrostatic air cleaners will remove dust, dirt, and some other pollutants from the air, but are useless in removing unwanted gases.

You should never smoke around your orchids (they might pick up the nasty habit the next thing you know, they will demand to be watered with Jim Beam and start hanging out with questionable Dandelions - OK, just kidding).

If you do smoke, be sure you wash your hands before handling your orchids to prevent spreading a deadly virus called Tobacco Mosaic Virus from your nasty cigarettes to your beautiful orchids.

Ethylene gas can cause the orchid's sepal to wilt - especially in the Cattleya genus which can be effected by as little as one part of ethylene gas to 300,000,000 parts of air.

Ethylene gas is produced from incomplete combustion of petroleum products (coal, natural gas, gasoline, oil etc.) as well as being produced naturally by some fruits such as apples.

A basket of apples can generate enough ethylene gas to cause your flowers to wilt and turn black.

In a greenhouse, smog, if brought inside through ventilation, is very harmful to the blooms because ethylene gas is also present in smog.

If you live in a very smoggy area you may want to keep your windows closed on the days of high air pollution. Non-electric (gas, propane, etc.) heaters used in greenhouses should be vented to the outside.

NOTE:
If your orchid blooms suddenly wilt, it may not be due to bad air. If the bloom suddenly fades and the petals fold together and turn papery, an insect may have pollinated the blooms. Look to see if there is pollinium on the stigma.