Estimation of risk from exposure to ra don is based on epidem iological data in m i ners, and extrapol ated to the lo w exposure levels present in hom es.

Bronchial epithelial cells in m i ners m a y ha ve been traversed by several particles in a short period.

For the individual in a domestic radon situ ation, it is highl y unlikely tha t any cell will be traversed by m o re than one al pha particle in an entire lifetime.

Objectives:

 Investigate the onco g enic effects of exactly one alpha particle.

 Co mpare the effects of exactly one to a Poisson mean of one alpha particle.

 This is a check on whether the cells receiving m o re than one particle dom inate the response.

 “Control” study: com p arison of exactly four with a Poisson mean of four particles.

The mi crobeam facil ity at Colum b ia.

Continuous im provements in autom a tion ma ke it possible to irradiate ~ 3000 cells per hour.

“Broad beam” studies produce the statis tical Poisson distri buti on of parti c le traversals.

For a m ean of 1 part icle:

 37% will see 0 traversals

 37% will see 1 traversals

 26% will see 2 or m o re traversals

The average dose wi ll be the same as the group that all received exactly one

traversal.

Cells are s t ained wit h a Ho ech st dye to visualize the nucleus for targeting. Broad beam groups are also stained as a control.

Transformation was judged by m i croscopic inspection of colonies aft e r 7 weeks of growth. Methods previously reported. Such transform e d cel ls produce tum o rs when injected into animals.

Transformed cells lose contact inhibition.

[Hall, 2000]

Transformed cells lose anchorage dependence.

[Hall, 2000]

Transformation frequency/10 4 surviving cells.

 Control rates are sim i lar.

 Broad beam results are greater t h an m i crobeam results only at 1 particle.

 At 2, 4, and 8 particles the m i crobeam results are th e same (within the error bars) as the broadbeam results.

 Cells traversed by exactly one alpha par ticle show a lower risk than those receiving a mean of one.

 Exactly one alpha p a rticle is not si gnificantly differe nt than control s receiving zero!!

 This im plies that the majority of tran sformed cells resulting from a mean of one traversal m u st come fro m the subpopu lation tha t received m o re than one traversal.

 If traversal by only one particle does not significantly raise the risk of oncogenic transform a tion, estimations by extrapolation from higher doses received b y min e rs will overestimat e the risk.

Previous work by this group with this cell line

 Exact numbers of alpha particles: survi v al and m u tation endpoi nts.

 Cytoplasmic irradiation.

 Now carried out to 20 particles per nucleus.

 D 0 is approxim a tely 3.6 al pha particle traversals.

 Survi v al after 20 particles through the nucleus is ~ 1%.

A L cells: ham s ter with one human chro m o so me that expresses cell surface markers for com p lement inactivation.

 ~500 cells plated in irradiation dish

 At random, 5, 10 or 20% of the cells in the dish irradi ated with 20 alpha particles each

 >99% of the survivi ng cells are unirradi ated, but the m u tation rate is 3 times higher tha n control.

 Bystander effect in cells not traversed by alpha particles.

 The effect appears to “saturate”.

 For the population where 20% of the cells were exposed to 20 alpha particles, 99.8% of the progeny are cal culated to be from unirradiated cells.

 The mut a tional spectra ar e clearly different from those that occur spontaneously.

 The bystander m u tagenicity was not affected by ad dition of DMSO.

 The effect is not due to a free radical ROS .

 Lindane blocks gap junctions a nd reduces the bystander effect.

 Cell-cell contact plays an im portant role in this effect.