Ideal humidity is at least 55 per cent; anything below 50 per cent is dangerous. Once or twice a year in London, the humidity goes down to the high 20s, but in Canada and the Nordic countries, true of Finland too, it can get down to single figures.
Musical instruments do not respond well to lack of humidity, and dry heat is the worst (as in mid-West USA, the Middle East or the Nordic region). Homes or schools with central heating at full blast can be very harmful. The wood shrinks when it is dry and when it cannot shrink any further, it cracks. A humidifier in the room where the instrument is kept is worth the cost, but it must be kept topped up with water during dry spells.
For travelling, a Trophy Paganini humidifier is useful and it is better to over-use it than under-use. It is, essentially, a long sponge inside a tube and held in the instrument through the f-hole. Soak the humidifier in water once a day during the summer. Another old-fashioned tip, that has some effect, is to keep a fairly fresh slice of potato in the case.
There is little you can do about damp heat, as in New York or Malaysia. You can get dehumidifying silica gel crystals, which are effective.