Inhalation or exposure to negative ions is a safe practice for humans, as these particles are already present in our environment from sources such as sunlight, lighting, and waterfalls. However, the ozone production of ionic air purifiers can be hazardous to our health. Inhaling ozone, even in small amounts, can cause throat irritation, coughing, chest pain, and difficulty breathing. It can also increase the risk of respiratory infections.
Ionizer air purifiers are generally safe because they don't produce enough energy to be dangerous. Despite the conflicting data surrounding ionizers, I usually keep mine turned off. I have a Therapure TPP300D with UV+ ionizer that runs for 8 hours every day in my living room. Air ionizers are designed to help provide cleaner air by releasing ionized particles.
To protect myself from second and third hand smoking from my roommate, I now plan to keep my ionizer turned on for only 30-40 minutes when no one is in the room and then turn it off before anyone enters. This way I can enjoy the benefits of cleaner air without risking my health.