Humidity, which is the amount of water vapor in the air, is measured by percentage. A comfortable indoor humidity level in the summer is between 40 percent and 50 percent. In winter, indoor humidity is usually between 30 percent and 40 percent. The humidity in a basement is higher than the floors above it because moisture is heavier than air and will fall to the lowest point. You want to dehumidify a basement apartment not just for comfort, but to reduce the chance that mildew will form.
Purchase a hygrometer to measure the humidity in your basement apartment. Hygrometers are available at hardware stores or home and garden centers for less than $20 and may have a thermometer included.
Seal around doors and windows to restrict the flow of humid outdoor air into your apartment.
Check the dryer vent. Make sure it is venting properly and is sealed well so moisture from the drying clothes isn't contributing to the humidity in the apartment.
Take baths instead of showers and run the exhaust fan to reduce steam. Use the microwave to cook instead of the stove to reduce humidity from cooking. Wash dishes by hand instead of using the dishwasher. If rain gutters are draining against the building, check to see if they can be routed to drain away from the building.
Measure the square footage of your apartment and purchase an air conditioner capable of cooling an area of that size. The air conditioner will need to be vented outdoors, so it should be installed through a window or a wall that has an area above ground level.
Use a dehumidifier to augment or instead of the air conditioner. The dehumidifier will not cool the air, but by pulling moisture from the air it will seem cooler. A dehumidifier deposits water pulled from the air into a container that periodically needs to be emptied. Alternatively, there are dehumidifiers available with a hose to drain them automatically. You would need to place that type of dehumidifier next to a floor drain or over a sink.
Place floor fans in areas where there is little or no air circulation, like a bedroom at the end of a hall. The object is to keep air moving toward the dehumidifier or air conditioning, and also to reduce the chance of mildew forming. On days when it is cool outside, place fans in the open windows facing out.