Research Highlights

Environmental Factors and Material Type Impact the Persistence of Ricin and Vaccinia

EPA is investigating the persistence (attenuation) of biological organisms and toxins on indoor building materials under various temperature and relative humidity conditions consistent with those achieved using heating, ventilation, and air conditioning (HVAC) systems. It is important to know whether environmental conditions can impact the persistence of biological agents in order to properly structure first-responder assessments, conduct risk assessments, develop remediation strategies, and interpret results from fumigant, liquid, or foam decontamination technology evaluations. Furthermore, decontamination of indoor surfaces following the intentional release of a biological agent may be enhanced by manipulating environmental conditions, such as temperature or relative humidity.

In the investigation report entitled Impact of Temperature and Humidity on the Persistence of Vaccinia Virus and Ricin Toxin on Indoor Surfaces, persistence was evaluated as a function of biological agent type (ricin toxin and vaccinia virus), building material surface (galvanized metal and painted concrete), and environmental conditions (temperature and relative humidity). The biological agents were selected based upon a review of available information and other ongoing research and assessment efforts.

Investigation Process

Persistence of biological threat agents exposed to known environmental conditions can be evaluated by comparing the temporal reduction in toxin bioactivity or organism viability following inoculation onto indoor building material coupons. In this investigation, coupons of galvanized metal and painted concrete were inoculated with ricin (25 µg) or vaccinia (6.76E+07 plaque-forming units [PFUs]) and placed in test conditions of:

• ~20°C and 40%–70% relative humidity (i.e., ambient)
• 30°C at high relative humidity (>70%)
• 30°C at low relative humidity (<40%)

The effect of these conditions on each agent and material combination was evaluated over a period of 14 days. Specifically, the extractable amount of ricin (measured as toxin bioactivity) or vaccinia (measured as PFUs) remaining on the coupons was measured on Days 1, 3, 9 and 14.

Investigation Results

The results of this investigation show that relative humidity, indoor material type, and time after application of the agent to the indoor material affect the persistence of ricin and vaccinia. Both ricin and vaccinia have greater persistence on galvanized metal and painted concrete under low relative humidity conditions than under high relative humidity conditions.

The results also suggest that the rate loss of ricin and vaccinia (i.e., the persistence at a given point in time) depends on the type of material onto which the biological agent is applied. For example, under ambient conditions, residual amounts of both ricin and vaccinia measured on Day 3 were very different for the two building materials.

Depending on the type of indoor material, elevated relative humidity may be useful for lowering the cytotoxicity of ricin before the application of a decontamination technology. Ricin is more persistent, for both galvanized metal and painted concrete, at low relative humidity. Ricin was persistent on painted concrete retaining substantial extractable cytotoxicity after Day 14, even at high relative humidity. For ricin on galvanized metal, little or no ricin cytotoxicity was observed by Day 14 at high relative humidity.

Table 1. Extractable Amounts of Ricin or Vaccinia from Different Materials at Low and High Relative Humidity Conditions

^aTemperature was constant at 30°C.
^bInoculation of 25 μg ricin and 6.76E+07 PFU vaccinia.

Elevated relative humidity may also be useful for inactivating the vaccinia virus before applying a decontamination technology. Vaccinia virus persisted on painted concrete or galvanized metal at Day 14 at low relative humidity. However, at high relative humidity, no viable vaccinia virus was extracted from galvanized metal on Day 3 or after nor from painted concrete on Day 9 or 14. Based on the results of this investigation, it is possible that elevated relative humidity will also have a comparable impact on the viability of Variola or other pox viruses on metal and painted concrete substrates.

Table 1 provides summary results for relative humidity. Although temperature was evaluated, this investigation did not show, but cannot rule out, the modest effects of temperature differences on the persistence of ricin and vaccinia. It is possible that the effects of temperature were masked by the significant effects of relative humidity.

Contact: Shawn Ryan

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