It is a complex task to find optimal protective clothing to prevent the spread of
Ebola virus disease (Martin-Moreno et al., 2014; Ryschon, 2014). The fear of getting
infected is an obstacle for recruiting healthcare workers. In addition, the current
design of protective clothing might curtail their working capacity severely in the
hot and humid climate of West Africa and, in addition, paradoxically increase the
risk of infection. Emergency work in full protective clothing including respiratory
mask may lead to extreme heat stress in the hot climates resulting in shortened work
time, dehydration, reduced professional judgement, and exhaustion. This increases
risk of infection of health stuff (WHO, 2014).
In Monrovia, Liberia, daytime maximum temperatures in the end of the year often reach
30–31°C, and the temperatures will be higher January to May, the hot season (Kjellstrom
et al., 2014; http://climatechip.org/). In order to manage this heat stress, the workers
need breaks (Kjellstrom et al., 2009). This leads to a frequent need to remove the
protective gear, which involves an increased risk of infection. The multiple steps
to remove the suit can take up to 30min (Kitamura, 2014).
The modified Predicted Heat Strain (ISO 7933, 2004) model was used to indicate the
expected work times (Fig. 1). The estimation was made based on the following assumptions.
Standard man was chosen for the model calculations. Medium heavy activity (300W) was
taken as the average work rate. The core temperature limit to cease such emergency
work was set to 38.5°C. Three clothing types with different moisture permeability
(i
m) were selected for comparison: an impermeable outer layer (i
m = 0.00), a semipermeable outer layer (i
m = 0.07), and a relatively tight but still permeable outer layer (i
m = 0.20). The basic clothing insulation in all cases was theoretically taken as 1
clo (0.155 m2K W−1) for comparative purposes. In all air temperature conditions, the
other environmental factors were kept constant. Ambient water vapour pressure was
set to 3.0 kPa, air velocity/body motion was 1 m s−1, and there was assumed no radiation
effect present (work indoors or in shade).
1
Continuous work times for a work rate of 300W at different air temperatures before
reaching a core temperature limit at 38.5°C in clothing with different moisture permeability
(i
m).
The chosen work load in impermeable and semipermeable clothing allows 40min or shorter
exposure during the hottest periods (Fig. 1) until the core temperature exceeds the
suggested safe limit for occupational exposure. Higher core temperature is associated
with decreased mental performance and increased misjudgement and mistakes (O’Neal
and Bishop, 2010).
Maximizing the moisture permeability and minimizing the clothing layers worn beneath
the protective gear, provided that it should be resistant to penetration by body fluids,
is a simple way of preventing heat stress and increasing the time spent inside the
gear. However, dehydration and water intake must also be considered during extended
exposures. A heat stress management program including rehydration should be an essential
part of the overall health and safety program in any case.
A desirable addition would be personal cooling used inside the protective clothing,
such as cooling vests with ice or phase change materials (PCMs; Gao, 2014) or filtered
ventilated coveralls (Kuklane et al., 2012). This may prolong working time to about
2h and reduce the number of gear changes per day. With 2-h work time in protective
gear, the number of required personnel could be halved with possible decrease in contaminated
waste. The final choice of the cooling method depends on specific air temperature
and humidity. Increasing air temperature and, especially, humidity do reduce the effectiveness
of air cooling and increase the benefits of PCM products.
The use of PCMs requires freezers or cool areas for solidification after use. Cooling
vests with ice are the cheapest and electricity for freezers is required. Power is
one of the basic resources to provide healthcare and to cope with epidemics. Otherwise,
the other types of PCM, e.g. Glauber’s salt or organic hydrocarbons/wax, with melting/solidifying
temperature at about 28°C are available. For workers’ recovery after heat exposure,
a room with air temperature below 27°C is recommended. The room or connected facilities
could be used for PCM solidification storage. If still unavailable, then the melted
PCM can be solidified in a relatively cooler water bath (using underground/well water,
etc.), in an underground cave or in a cooler area during night. The higher the melting
temperatures are, the less effective cooling is. However, if the temperature gradient
is about 6°C or greater, the PCM can still provide a cooling effect.
Considering cooling effect in ventilated garments, the provided air flow should be
above 100 l min−1. There are filtered fan systems available on the market that manage
the flows up to and above 200 l min−1 with the battery power lasting at least 5–8h
(recharging takes about 2h). Ventilated systems (positive pressure suits) may allow
even drinking water in the suit and that may prolong the work time even more.
Table 1 gives a rough cost comparison of the present and a possible future protective
clothing system based on 1-day (8-h) shift. It takes into account only the equipment
cost. Estimation is based on the work time predictions given in Fig. 1 for the hottest
work periods, i.e. 30min for the impermeable set and 2h for the new system that prolongs
work period by higher permeability or by use of a cooling device. In both cases, similar
final core temperatures are expected to limit the exposure. Also, it is expected that
both sets take 30min for dressing, 30min for undressing, and require 30min for recovery
between the work periods. As it can be seen the equipment cost of a new, theoretically
even a 10 times more expensive solution is almost 3 times higher for a day.
Table 1.
Comparison of the equipment cost of the present and a possible, 10 times more expensive
protective clothing system based on 1-day (8-h) shift. Assumed work time is 30min
for present and 2h for the new system. In both cases, expected donning, doffing, and
recovery periods are 30min each.
Present set
New set
Work time under a work session (h)
0.5
2
Workers needed to cover a continuous 8-h work shift (nr.)
4
2
Approximate cost per set ($)
90
900
Number of sets needed per 8-h shift (nr.)
16
4
Total PPE (personal protective equipment) cost per 8-h shift ($)
1440
3600
Simultaneously, there are also other benefits with an actively cooling clothing system.
The personnel need to cover one workstation is halved. The personnel have even extra
time (about 30min) between the shifts to help with any other tasks or for additional
recovery. Due to fewer times of dressing–undressing (16 + 16 times 30min versus 4
+ 4 times 30min for present respective new system), there is also less need for assistance
and disinfection during these periods. There will be less contaminated waste or fewer
amounts of products to be cleaned. The new systems are meant to be reusable (extra
costs for decontamination procedures have to be considered) compared to present, supposedly
disposable systems, and already 2.5 times reuse will even up the equipment costs at
the estimated prices. Infection risks are diminished due to the reduced need for undressing
and cleaning procedures.
In conclusion, reducing the risk of infection among the front-line healthcare workers
and allowing a doubling of their work capacity could be a critical factor to successfully
contain the epidemic. Considering that this epidemic is not the last, and with warmer
climate both the epidemics are expected becoming more frequent, and conditions to
fight them more severe (IPCC, 2013), then the testing and evaluation for selection
of the optimal equipment is required long before missions are set out.