Placing one lamp in each room is beneficial. It’s safer to use a 7w -15w bulb which can be left on all the time.   If you use a higher watt bulb, make sure you keep an eye on it; could be a fire hazard.  Follow manufacturer instructions.  Benefits of salt lamps:

• Negative ions generated from the lamp drag positive ions from the air. Positive ions are air pollutants.
• May Reduce the symptoms of Asthma & Allergies
• Boosts Energy levels
• Natural Air Purifier
• Helps reduce Chronic cough
• Reduces Static Electricity
• Improves quality of sleep
• Reduction of Electromagnetic Radiation
• Mood Booster and Stress Reliever
• Environmentally friendly
• Helps with SAD (seasonal affective disorder) (8)