Many places are in the grip of a heat wave - and nothing looks more inviting than a visit to the local swimming pool. But be careful - some use disinfectants that can cause asthma and bladder cancer, researchers warn this week. The problem isn't just with the disinfectant, it's the chemical reaction that occurs when it is mixed with cosmetics and sunscreen creams on the swimmers. Many consumer product are nitrogen-based, and the reaction when mixed with pool disinfectant can produce a toxic agent that can mutate genes, cause birth defects, accelerate the ageing process, cause respiratory problems such as asthma, and even cause cancer. The solution? Everyone either showers before getting in the water, or chooses a pool that doesn't use disinfectant for filtering the water, say researchers from the University of Illinois. (Source: Environmental Science & Technology, 2010; 44: 3527-32).