There's acutely unhealthy for sensitive groups and there's long term unhealthy for anyone.

Convincing reasons exist to think that significantly lower levels than are acutely unhealthy have long term health impacts. On top of that, you don't really need a sensor to know whether or not your air has particulate levels that are acutely unhealthy, you just know it.

If you don't care or only want to know if your air is quite bad, sure, buy one of these. I don't think it's a reasonable level, but you do you.