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 l/min,¹ the number of particles/g is1.7×10¹⁰,^11,20 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 on 20 000 μg/petri dish (90 mm diameter), the highest concentration reliably recorded.⁴⁰ This concentration was found only in one trial¹ and equates to 110 μg/cm².

A total of 42 trials with dissemination by air are documented, of which 29 took place over land, 11 over the sea, and two over both land and sea (table 2). The amount of ZnCdS disseminated could be calculated for 35 of the 42 trials, with a mean of 70 kg, range 5–139 kg, and a total amount disseminated in 35 trials of 2446 kg.

The highest particle count (4315) was found at Dorchester after dissemination over the English Channel in 1959, with 127 kg ZnCdS released from a Venturi operated dispenser mounted in a Valetta aircraft, with the flight path finishing close inshore just south of Swanage. Counts over 1000 were also recorded over Cardington in August 1957 (particle count =3403) and November 1957 (1070), and at Silloth (1591), the start of a flight path over northern England and the North Sea.

There were nine trials with dissemination from ships (table 3). The highest particle count (1676) was recorded at Dorchester when 11 kg ZnCdS was discharged 18 miles south of Portland Bill. The second set of trials, conducted in the English Channel in January 1963, concerned the penetration of ships by aerosol, and no land based monitoring was conducted. In total 250 kg ZnCdS was released from nine trials.

An estimated 2700 kg was released from 35 air disseminated and nine ship disseminated trials, and 250 kg from land based trials. Assuming that the mean of 70 kg was released from each of the seven undocumented long range trials, an estimated 3450 kg was disseminated in total. The programmes dated 1960–4, for which further data are not available, are summarised in table 4. Altogether 28 trials were proposed, but three were small and are not considered further. Proposed release of ZnCdS in the remaining 25 trials was 1387 kg. Assuming these trials took place, this would have resulted in a total ZnCdS disseminated during 8 years of about 4850 kg (1503 kg Cd).

The largest estimated dose received by any person during any one trial was 0.42 μg ZnCdS (0.13 μg Cd) (table 2). Assuming a total of 76 long range dissemination trials (51 trials in tables 2 and 3 and 25 trials in table 4), and a “worst case” assumption that at each trial a person received the maximum dose (0.13 μg Cd, calculated from the Dorchester measurements), then the maximum total dose over an 8 year period was about 10 μg Cd.

DISCUSSION

Zinc cadmium sulfide is a sintered, crystalline compound the photosensitivity of which makes it useful for photography, and it is thought to be safe in this role.⁴¹ However, Cd accumulates in the body, especially in the kidneys and to a lesser extent in the liver. There is uncertainty about the fate of ZnCdS when it is breathed deep into the lungs. Cadmium ions are carcinogenic in the alveolar cells, but the insolubility of ZnCdS probably reduces this potential. About 5% of ingested Cd⁴² and up to 57% of inhaled Cd⁴³ is absorbed, bound to albumen or to metallothionein. Initially stored in the liver, Cd metallothionein enters the kidneys, where the lysosomes in the proximal tubules release the Cd, impairing tubule function. The estimated 1.5 tonnes total Cd dissemination by the Ministry of Defence compares with an estimated 15 tonnes Cd released into the atmosphere from United Kingdom industry annually.⁴⁴ Thus, the United Kingdom trials increased the atmospheric Cd load by about 1.2%.

The estimated maximum inhaled dose from any one trial was 0.13 μg Cd, which is several orders of magnitude lower than the Cd concentration in air (100 μg Cd/l) required to cause lung damage in mammals.⁴⁵ Our “worst case” estimate of cumulative personal dose received from all trials was about 10 μg, which is of the same order as the estimated total Cd inhalation in North American cities affected by the tests (24.4, 14.5, and 6.8 μg in St Louis, Winnipeg, and Minneapolis, respectively).¹

The National Research Council estimated that the average yearly inhalation dose of Cd from other sources was 30–250 μg, with smoking adding 2–4 μg/20 cigarettes.¹ Thus, the “worst case” dose in the United Kingdom equated to the amount of Cd inhaled in an urban environment in 12–100 days, or from smoking about 100 cigarettes.

Being insoluble, CdS has a lower bioavailability when inhaled than soluble Cd compounds.⁴⁶ However, the possibility of photodegradation of a proportion of the ZnCdS to soluble forms—such as ZnCdSO₄—cannot be ruled out. Assuming renal accumulation of 35% of Cd cleared by the lungs, and lung clearance of 56% of inhaled Cd,⁴⁶ a 10 μg inhaled dose could result in an additional renal burden of about 2 μg Cd. As the currently accepted critical limit of Cd in the renal cortex is 200 μg/g,⁴⁷ any additional renal burden was negligible.

The immediate contamination from ground based dissemination was probably not a serious problem as most releases were on airfields, which were not used for food production. There are two perceived risks of long term soil contamination, to garden plants subsequently grown on the site which may accumulate and contaminate consumers, and to children consuming contaminated soil. No long term adverse effects through garden plants have been established from Cd contaminated garden soil at Shipham.⁴⁸

United Kingdom regulations for sewage sludge application provide guidance on safe Cd concentrations in soils.⁴⁹ Up to 3 μg Cd/g soil dry matter in the top 20 cm is permitted, with a maximum deposition of 1.5 μg Cd/cm²/10 years. The deposition rate of 110 μg/cm² at Beaulieu exceeds this limit, but is not illegal as it does not arise from sewage. Assuming a soil bulk density of 1.5 g/cm³, the top 20 cm of soil contains at most an additional 3.3 μg Cd/g dry matter from the Beaulieu test. Added to the median Cd soil concentration (0.7 μg/g), the expected maximum is 4 μg/g dry matter, above the permitted maximum for sewage sludge application, although the area affected was small (table 1).

The risk of soil consumption by children can be determined from the estimated lowest observed adverse effect level (to produce acute gastrointestinal symptoms) for a single dose of ingested Cd, estimated at 43 μg/kg body weight.⁵⁰ A 10 kg child would need to eat 108 g dry matter of soil with 4 μg Cd/g to reach this dose. The background intake of Cd (12–84 μg/day in urban areas, 10–60 μg/day from food and water) is much higher than from contaminated soil.

The greatest risk of ground contamination was probably from material washed off the dispenser in the field before it was returned to headquarters, as operators were instructed that “gross contamination around the dispenser on the trailer will be brushed off and washed down before departing”. However, the sites are not sufficiently precisely identified in the reports to permit remedial action, and in any case, in the intervening 35–48 years it is to be expected that some of the ZnCdS will have been subjected to chemical weathering and transmutation, broken down by bacteria, and leached from the surface soil. Any material that resisted breakdown for this period should be so insoluble as not to pose a health hazard.

A further possible risk was to four Ministry of Defence officials that operated disseminators, as the respirators may not have protected them against the particles, which had median diameter 1.5 μm and density 4 g/cm³. Two disseminators have survived and are in good health, one died aged 73 of a heart attack, and the final operator could not be definitively traced but is thought to be alive. The building on the Porton Range used for storing ZnCdS was destroyed at the end of the programme. Operators probably risked damage to eyes and exposed skin from ultraviolet light when counting ZnCdS particles, but this seemingly did not happen.

In conclusion, between 1953 and 1964, ZnCdS particles were disseminated from static sources, vehicles, ships, and aircraft by the Ministry of Defence. In long range trials, the inhaled dose during the most severe case was small (0.13 μg Cd). Land based trials did not significantly add to soil Cd, except close to the dissemination point. Risk to the public is therefore considered negligible. People involved in dissemination, who would have been exposed to large amounts of Cd, had no ill effects.