The effects of long-term exposure to high and low humidity on thin-film humidity sensors were studied. Five sensors were calibrated, then exposed to high humidity for four months, resulting in a slight increase in readings. Interestingly, the cheaper sensors showed a larger increase. After exposure to low humidity for another four months, the readings decreased, indicating recovery from the high-humidity effects. This study shows that thin-film humidity sensors can be used in high-humidity environments like greenhouses and food factories for extended periods.