4. Second hand smoke

Why it's bad: Passive smoking through second hand smoke can be extremely harmful for anyone who suffers from asthma or other respiratory problems, especially for children.

How to tackle it: This is one that you can easily control -- don't pass your second hand smoke on or let others smoke in your home.

5. Radon

Why it's bad: Although radon is 100% natural, in many countries this colorless, odorless, tasteless radioactive gas is the second most important cause of lung cancer after smoking. For most people, the greatest exposure comes from home. But don't fret just yet -- radon levels vary significantly among different locations, depending on the concentrations in the underlying rocks and soils. Radon seeps into homes through cracks and gaps in the walls and floors, sumps and drains, meaning radon levels are usually higher in basements and cellars; and as radon diffuses upwards, its concentration typically falls on the upper floors.

How to tackle it: To find out if radon is a problem in your house, you can purchase a test kit -- it costs less than you'd expect and can quickly tell you how high the radon level is. If it's above what's considered 'safe', the EPA recommends getting a professional in to install a radon mitigation system to control the levels. Simple solutions like keeping your house well-ventilated and sealing floors and walls can also help reduce indoor radon levels.

General tips for improving air quality

• Have good ventilation. Opening the windows brings in cleaner outdoor air and improves your indoor air quality. Do it for 10 to 15 minutes a day.
• Remove or reduce the culprits, for example carpeting, water leaks, and half-used cans of paint.
• Use natural cleaners -- vinegar, boric acid and baking soda can be used to clean your home without VOCs. Mix vinegar and water to wipe surfaces, use boric acid to clean toilets and baking soda to scour.
• Vacuum slowly. Vacuuming in a slow way captures dust the most efficiently -- the faster you go, the more dust you'll raise. When emptying the vacuum cleaner bag, step outside the house to do it.