Comment by o11c

An air purifier. Note that they'll try to sell you on an infinite supply of replacement filters, but if you do the maintenance aggressively, you keep much of the benefit even just reusing the same filter.

Besides that, they say to never cheap out on anything that goes between you and the ground. Car tires appear to be a scam but most everything else works. Advice for specific items will be highly specific to where you are and what you do, but a general observation is: an object stops being effective long before it breaks entirely.

Since we're computer people, I would point out that there's a huge difference between a $150 ergonomic office chair and a $50 office chair. (though changing position is also important)