No detector is a substitute to regularly having your appliances checked anyway. However detectors, even cheaper ones, are better than nothing and most people will not pay or be able to afford expensive ones. Just replaced mine with a set of them, all will fire at 50ppm as per standards this side of the pond (EN50291-2010). They are for domestic use and yes won't detect very low levels to avoid being a nuisance and that's important because like with fire alarms a lot of people stupidly take the batteries out to stop them being annoying! Though some people with medical conditions need more sensitive ones. However most people aren't going to feel effects, even long term, at 10ppm.