A hypothetical stochastic mechanism of radiation effects in single cells.

This paper is a sequel to an earlier one (Neyman & Puri 1976), which deals with a hypothetical structural stochastic model of radiation effects in living cells. This model incorporates two important details of the mechanism overlooked in its mathematical treatment by the previous workers. The first is that the passage of a single "primary' radiation particle generates a "cluster' of secondaries which can produce "hits' that damage the living cells. The second detail concerns the time scales of radiation damage and of the subsequent repair. The events of arrival of a primary particle, its generation of secondary particles and their causing "hits' on the sensitive targets within the cells, all occur for all practical purposes instantly. On the other hand the subsequent changes in the damaged cell, such as repair, appear to require measurable amounts of time. While the biological and physical justifications for some of the underlying assumptions of the model were discussed in the previous paper referred to above, the present paper is concerned mainly with the mathematical details and also with how the model attempts to explain some of the empirical findings available in the literature.

Public health hazards from electricity-producing plants.

When a new electricity-producing plant is to be built in a given locality it is natural to take into account the public health consequences of the normal operation of each type of plant contemplated. Here, the fossil-burning plants and nuclear facilities come under consideration. I have attempted to show that, in spite of the many important studies performed, there is currently no reliable methodology to estimate how many more cancer cases, and how many more heart attacks and other diseases have to be anticipated as a consequence of the normal operation of this or that type of electric generator. In part, this is because the currently available estimates of radiation effects on humans are based on extrapolations from studies of two kinds. Those of one kind may be exemplified by studies of atomic bomb casualties in Hiroshima and Nagasaki. The other kind are laboratory experiments with lower animals, frequently mice. The unreliability of both kinds of extrapolations is connected with the following circumstances: (i) The omnipresent troublesome phenomenon of competing risks. (ii) The dependence of health effects of a given noxious agent on the preexisting local pollution. (iii) The dependence of health effects not only on the "dose" of an agent, but also on the rate at which the agent is administered. (iv) The noted difficulties of making extrapolations from one mammal to another. Our obtaining reliable estimates of the public health effects of extra pollution from new industrial plants would seem to depend on a large multipollutant and multilocality epidemiological study being conducted--one requiring the cooperative effort of several governmental agencies. However, a much easier study of certain developments in the vicinity of Rocky Flats, Colorado, might provide important direct information on health phenomena as they occur in real life.

A structural model of radiation effects in living cells.

The chance mechanism of cell damage and of repair in the course of irradiation involves two details familiar to biologists that thus far seem to have been overlooked in mathematical treatment. One of these details is that, generally, the passage of a single "primary" radiation particle generates a "cluster" of secondaries which can produce "hits" that damage the living cell. With high linear energy transfer, each cluster contains very many secondary particles. With low linear energy transfer, the number of secondaries per cluster is generally small. The second overlooked detail of the chance mechanism is concerned with what may be called the time scales of radiation damage and of the subsequent repair. The generation of a cluster of secondary particles and the possible hits occur so rapidly that, for all practical purposes, they may be considered as occurring instantly. On the other hand, the subsequent changes in the damaged cells appear to require measurable amounts of time. The constructed stochastic model embodies these details, the clustering of secondary particles and the time scale difference. The results explain certain details of observed phenomena.

Downwind and upwind effects in the Arizona cloud-seeding experiment.

The principal subject of this report is a comparison of precipitation on days with seeding with that without seeding, averaged over those rain gauges that on each particular day were "downwind," "upwind," or to the sides. Two estimates of relevant wind directions are used, based on successive radiosondes at Tucson that bracketed the scheduled time of seeding. By use of these radiosondes, the apparent effects of seeding on rain in downwind localities 90-180 miles (145-290 km) away from target were found to be an apparent 45% loss of rain (P = 0.002) and an apparent 34% loss of rain (P = 0.028), respectively. Other results indicate considerable geographic heterogeneity.

Re-evaluation of the Arizona cloud-seeding experiment.

The apparent effect of cloud seeding on the average 24-hr precipitation in the Santa Catalina Mountains during the two programs of the 7-year-long Arizona experiment was found to be a 30% loss of rain (P = 0.06). Considering rainy days only, the apparent effect is a 34% loss of rain (P = 0.03). On South-East days the apparent loss was 40% (P = 0.03). The analysis of the diurnal variation in the amounts of hourly precipitation brought out two suggestions: (i) more active silver iodide enters the clouds through seeding at their bases than at the -6 degrees C level; (ii) the population of experimental days includes two categories with opposite responses to seeding: augmentations of rain in one case and losses in the other. These suggestions require independent confirmation.

Hypothetical explanations of the negative apparent effects of cloud seeding in the whitetop experiment.

In order to explain the apparent losses of rain ascribable to seeding at the Whitetop trial, particularly large and highly significant in the stratum E (but not in the opposite stratum W) of experimental days, it has been hypothesized that seeding causes widespread cloudiness and subsequent lowering of ground temperatures. This hypothesis is flatly contradicted by the observations: the seeded E-days (but not W-days) were uniformly less cloudy and hotter than those without seeding. Curiously, these differences prevailed not only from the scheduled time of seeding but also for several hours beforehand. The average rainfall for the 10 hr that preceded the time of seeding was investigated in eight "cells", defined by the day's wind direction to be downwind, upwind, and to the sides and "far" and "near" the center of seeding. Highly significant decreases were found in the far-upwind and far-left cells, indicating an earlymorning disparity between those E-days that later were declared as experimental to be seeded and those E-days that were declared as experimental not to be seeded. This disparity, difficult to explain by chance variation, suggests that particular caution be used in treating differences in the rainfall between seeded and not-seeded days in the Whitetop trial as having been caused by seeding.

Evidence of widespread effects of cloud seeding at two Arizona experiments.

The average effect of two cloud seeding experiments (1957-1960; 1961, 1962, and 1964) over the Santa Catalina Mountains, Arizona, on the 24-hr precipitation at Walnut Gulch, 65 miles away, was an apparent 40% loss of rainfall (P = 0.025) on seeded, as opposed to not-seeded, experimental days. Larger apparent losses, some highly significant, were found for experimental days on which Walnut Gulch was downwind from the seeding site (but not on upwind days), and also on "second days" of the randomized pairs (but not on "first days"). The timing of significant apparent effects indicated that the afternoon maximum of precipitation, which is very pronounced on days without seeding, is either absent or weakened on days with seeding. This phenomenon was observed earlier in a study of the Whitetop Project.

Further studies of the whitetop cloud-seeding experiment.

By means of two stratifications of experimental days of the Whitetop Project-into categories E and W and into categories air mass and frontal-the effects of cloud seeding on precipitation downwind, upwind, and to the sides, up to 180 miles from the seeding line, were investigated. No significant effects were found for W and frontal days. On the other hand, for E and airmass days, significant and highly significant apparent effects of seeding were found in all directions and in areas at distances up to 180 miles. All these significant apparent effects are negative, the largest of them indicating that seeding reduced precipitation to one quarter of its unseeded value.

Wind directions aloft and effects of seeding on precipitation in the whitetop experiment.

The subdivision of all the experimental days of the Whitetop project into two approximately equal groups, group W with predominantly westerly winds aloft and group E with frequent easterly winds, shows a remarkable difference in the apparent effect of seeding. On W days there was no detectable effect of seeding on rainfall. On E days with seeding, the average 24 hour precipitation in an area of about 100,000 square miles was significantly less than that without seeding by 46 per cent of the latter. The decrease resulted from a "decapitation" of the usual afternoon rise in rainfall. It may be significant that the afternoon maximum of natural precipitation on E days occurs some two hours later than on W days. If the actual cause of the differences in rainfall was seeding, then the loss of water resulting from operational, rather than experimental, seeding would have averaged eight million acre-feet per summer.

Timing of the apparent effects of cloud seeding.

The average hourly precipitation amounts, on 96 experimental days without cloud seeding in the Whitetop experiment, show a marked maximum between 4 and 7 o'clock in the afternoon, presumably reflecting the convection activity caused by heating of the ground occurring during an earlier period. No such maximum is observed on the 102 days with seeding. The hypothetical explanation presupposes that seeding with silver iodide creates early general cloudiness, which prevents ground temperatures from rising to levels usually attained on days without seeding. This hypothesis may explain not only the mechanism of the loss in rain in the Whitetop experiment, apparently induced by seeding, but also may explain certain phenomena noticed in the Grossversuch III experiment.

Areal spread of the effect of cloud seeding at the whitetop experiment.

With reference to arguments that weather modification technology is sufficiently advanced for the federal government to finance cloud-seeding operations as a means of alleviating water shortages, an analysis of the Whitetop rain stimulation experiment was performed. The average 24-hour precipitation in six concentric regions up to 180 miles from the center of the target on 102 days of cloud seeding was less than that on the 96 experimental days without seeding. For distances less than 30 miles, the apparent loss of rain due to seeding was 32 percent. With the increase in distance, this apparent loss decreased to a minimum of 9 percent for gages between 120 and 150 miles from the center. However, the 48 gages at distances between 150 and 180 miles showed a 22 percent apparent loss of rain due to seeding. The estimated average loss of rain within the whole region of about 100,000 square miles was 21 percent of what would have fallen without seeding. When a 5-year experiment, expected to produce a 5 to 10-percent increase, shows a 20-percent decrease in rainfall, the relevant technology does not appear reliable enough for practical use.