The risk to the United Kingdom population of zinc cadmium sulfide dispersion by the Ministry of Defence during the "Cold War". (Original Article).

Objectives: To estimate exposures to cadmium (Cd) received by the United Kingdom population as a result of the dispersion of zinc Cd sulfide (ZnCdS) by the Ministry of Defence between 1953 and 1964, as a simulator of biological warfare agents.

Methods: A retrospective risk assessment study was carried out on the United Kingdom population during the period 1953-64. This determined land and air dispersion of ZnCds over most of the United Kingdom, inhalation exposure of the United Kingdom population, soil contamination, and risks to personnel operating equipment that dispersed ZnCdS.

Results: About 4600 kg ZnCdS were dispersed from aircraft and ships, at times when the prevailing winds would allow large areas of the country to be covered. Cadmium released from 44 long range trials for which data are available, and extrapolated to a total of 76 trials to allow for trials with incomplete information, is about 1.2% of the estimated total release of Cd into the atmosphere over the same period. "Worst case" estimates are 10 [micro]g Cd inhaled over 8 years, equivalent to Cd inhaled in an urban environment in 12-100 days, or from smoking 100 cigarettes. A further 250 kg ZnCdS was dispersed from the land based sites, but significant soil contamination occurred only in limited areas, which were and have remained uninhabited. Of the four personnel involved in the dispersion procedures (who were probably exposed to much higher concentrations of Cd than people on the ground), none are suspected of having related illnesses.

Conclusion: Exposure to Cd from dissemination of ZnCdS during the "cold war" should not have resulted in adverse health effects in the United Kingdom population.

Early in the "cold war", from 1953 to 1964, a programme of zinc cadmium sulfide (ZnCdS) dispersion tests was conducted by the British Ministry of Defence, to simulate biological attack by communist forces. Discrete dissemination of toxic biological agents over the entire country was considered a high risk for an island such as Britain. Studies were conducted in Britain, America, and Scandinavia to determine whether small particles could be widely dispersed from aircraft or vehicles. Zinc cadmium sulfide was a suitable tracer, because it fluoresces under ultraviolet (UV) light and single particles could be counted on samplers around the country.

Initially various static disseminating and recording devices were tested at ground level, followed by release from vehicles and aircraft, from which the primary threat existed. Further studies investigated release from ships in the English Channel and the Irish Sea.

When information about the trials was released in the 1990s, public concern about health risks was voiced in America and Britain, but a toxicological assessment of the dispersion tests of ZnCdS carried out by the United States Army was reassuring. (1) We independently reviewed the United Kingdom trials, assessing possible human exposure to cadmium (Cd) (the presumed toxic component of ZnCdS), (1) and conducted a risk assessment in comparison with other sources of Cd.

METHODS

Field trial programmes, reports, and technical papers from Porton Down (2-36 37) detailing the United Kingdom trials (most of which are, or shortly will be, available through the Public Records Office) were provided by the Defence Evaluation and Research Agency. Planned field programmes, operation dates, dissemination routes, quantities of ZnCdS disseminated and recovered and locations of sampling stations were described. Two internal reviews were conducted. (38 39) Ground disseminating trials each released 0.4-9 kg ZnCdS from either a point source, to test equipment, or a vehicle, to simulate stealth attack. In early programmes fallout was measured in Petri dishes placed at about 2 m intervals in a grid around the disseminator.

Not all disseminations from aircraft and ships were comprehensively recorded. Some are reported more than once, and for others (approved in field programmes) no details are available. The aircraft trials over the sea were conducted 10-50 miles off the coast at an altitude of about 300 m. Particle fallout was monitored by cascade impactors or drum impactors located across the country. Widespread dissemination over hundreds of miles occurred.

The first trial using aircraft dissemination in 1956 released 12 kg ZnCdS over Porton from a hand fed Venturi unit below the aircraft. The last recorded aircraft dissemination in 1963 released 68 kg ZnCdS over 62 miles upwind and south west of Norwich. Other programmes, dated 1960-4, proposed dissemination by air at an unstated location (programme 23/60) and over Cardington (programmes 2/61, 24/62 and 10/63), Netheravon (programme 14/63) and Norwich (programme 2/64), but no details are available. Nine disseminations from ships are recorded in October and November 1959 and January 1963.

For the long range trials, total ZnCdS disseminated was estimated from dissemination rate and trial duration. The theoretical inhaled dose at the sampling point with maximum particle count (the dose received by a person at that point during the passage of the particle cloud) was calculated from the number of inhaled particles. The following assumptions were made: the breathing rate of an active adult is 16.6 1/min, (1) the number of particles/g is l.7x10 (10 11 20) 20 there was a 50% loss of fluorescence in particles due to exposure to sunlight, (38) and the ZnCdS was prepared from 60% ZnS and 40% CdS (31% cd). (38)

RESULTS

About 51 trials were conducted from ground based sources, mostly at Porton and local airfields, with an estimated total of 250 kg ZnCdS disseminated (table 1). Calculations of maximum fallout are based un 20 000 [micro]g/petri dish (90 mm diameter), the highest concentration reliably recorded. (40) This concentration was found only in one trial (1) and equates to 110 [micro]g/[cm.sup.2].

A total of 42 trials with dissemination by …