Complex sentence: Emissions from memory foam mattresses may cause more respiratory irritation than other mattresses; however mildew and house dust mites may not occur as frequently, so asthma attacks may be less frequent and severe.
Split into simpler sentences: Emissions from memory foam mattresses may cause more respiratory irritation than other mattresses. Mildew and house dust mites may not occur as frequently, so asthma attacks may be less frequent and severe.