Is it stupid to get an air purifier if I leave a window open?

To get the best out of an air purifier, it should be left on all the time and in a room without a fully-open door. Please note that purifiers like Airfree's should be left in-situ for 2-3 weeks before they will have full effect. If you're suffering from 'snuffles' and the like, it's quite possible that the problem isn't the outside air (FDA tests show that this is much better than indoor air, even in a city), but dust and dust mites. Mites and the dust they cause represent the most important household allergen in terms of people affected. In this case, an air purifier will create a much 'cleaner' atmosphere in your bedroom.

I’m guessing you want to continue keeping the window open, and it would help if you can rule out if Pollution is the cause of your sleeping problems. Your sleep specialist might also benefit from this information. There are two ways to do this:The quick and dirty way:Use publicly available information to arrive at the kind of pollutants that are present in your neighborhood. Information is available at http://www.baaqmd.gov/ website and also on https://www.airnow.gov/index.cfm?action=airnow.local_city&cityid=317. Details are also available on http://aqicn.org/map/california/san-francisco/san-francisco-arkansas-street/#@g/37.7547/-122.3883/10z including the specific pollutants in areas where data is available. Attaching screenshot from AQICN for today’s data in SF: Once you know the kind of pollutant that is creating poor air quality, you can take SPECIFIC action against that pollutant indoors. Air purifiers will remove Particulates (make sure you get one with HEPA filtration), Volatile Organic Compounds (filter with activated carbon) but not most other gaseous pollutants like CO, CO2, NOx, SOx, etc. In all likelihood, the air quality would be poor or good based on particulates alone, for which your air purifier will be effective with WINDOWS CLOSED.The correct and expensive way:If you aren’t happy with the QnD way, here’s the CnE way. Get the following: Particulate Air Quality monitor (eg. Dylos 1100 / 1700) CO and CO2 monitor NOx + SOx monitor Monitor for measuring hydrocarbons Monitor for Volatile Organics (also hydrocarbons) While you’re at it, a monitor for Radon (radioactive, present in homes) Get readings for your place of residence, and take corrective steps against whichever pollutant is above normal. The website links shared above will help in determining acceptable pollutant levels.Again, unless the source of pollution is indoors (pet dander, dust mites, fungii spores, etc.), you want to keep your WINDOWS CLOSED.Hope this helps. Do comment and let me know what worked for you!