Whether you should leave your air purifier on all day depends on your specific needs and the type of air purifier you have. In general, leaving your air purifier on all day can help to maintain a consistently clean and healthy indoor environment. However, there are a few factors to consider:

Energy Usage: Running an air purifier all day can use a significant amount of energy, which can increase your electricity bill. If energy usage is a concern, consider using your air purifier only when you are in the room or during times when indoor air quality is likely to be poor.

Noise Level: Some air purifiers can be quite noisy, which can be disruptive to sleep or work. Consider the noise level of your air purifier and whether it is appropriate to leave it on all day in your specific situation.

Filter Replacement: Air purifiers typically require regular filter replacements to maintain their effectiveness. Leaving your air purifier on all day can cause the filter to fill up more quickly, requiring more frequent replacement and higher expenditures.

Indoor Air Quality: If you live in an area with poor outdoor air quality or have indoor air quality issues like allergies or asthma, leaving your air purifier on all day can help to maintain a consistently clean and healthy indoor environment.

In general, leaving your air purifier on all day can be beneficial for maintaining indoor air quality, but it's important to consider the factors above and your specific needs before making a decision. If you choose to leave your air purifier on all day, be sure to follow the manufacturer's recommendations for filter replacement and maintenance to ensure optimal performance.