The potential for reactions among indoor pollutants to generate reactive and highly irritatingproducts is a reason to maintain adequate ventilation rates and clean ventilation filters.Terpenoid (from recirculated air, a scenario common in the tropics) captured by ventilationfilters can react relatively quickly with ozone which may lead to downstream air supply thatcontain oxidized terpenoid and this may be perceived to be less acceptable than outdoor air.The chemical composition (water solubility and chemical reactivity) of these particlesstrongly governs their toxicity.