constituents of ETS are likely to enter the lung of the exposed individual, while the hydrophilic vapor phase constituents are likely to be absorbed in the upper respiratory tract. Particles <2.5 µm (in this report referred to as respirable suspended particulates [RSP]) dominate the particulate phase of ETS and can be inhaled deeply into the lung.

Standard laboratory procedures have been established to assess the physicochemical properties of SS and MS. Research is needed to standardize both the collection and evaluation of ETS so that the effects of ETS can be studied in laboratories and in human populations.

The changes in distribution of particular constituents of ETS as the smoke ages in the indoor environment are largely unknown. For example, it is known that almost all of the nicotine shifts from the particulate phase in MS and fresh SS to the vapor phase in ETS. Consequently, indoor air-cleaning systems designed to remove particles will not greatly alter the nicotine exposure, but may alter the concentrations of other noxious or toxic components. Research is needed to determine the distribution of constituents in the particulate and vapor phases of aged ETS. Also, the efficiency of air-cleaning systems in removing the constituents needs to be studied.

Indoor radon comes from sources in the environment and decays to short-lived radon daughters, which may become bound to the RSP in ETS. However, some long-lived radon daughters come from tobacco itself. Research should be conducted on possible interactions between ETS and radon daughters, especially as radon daughters can adhere to RSP and increase the potential hazard of ETS.

There are currently no direct measures of the dose absorbed of ETS in a population under study. Exposures to ETS, however, can be assessed by questionnaires, air monitoring, modeling of concentrations, or biological markers. Future epidemiologic studies should incorporate into their design several of these exposure assessment methods in order to assess exposures to ETS more accurately and to estimate dose.