During winter, indoor air quality is very important and this can be achieved by minimal fluctuation rate of temperature as ventilation rate is restricted. This was well demonstrated in the present study as shown by the daily air temperature dynamics in the EPH (Figure 2). The outdoor temperature during the test period ranged from 5.5 to 6.9°C whilst the daily indoor temperature fluctuation was 12.4°C. This temperature difference has a great deviation when compared to the pigs tolerable daily temperature range of 4°C. Expectedly, the pigs will have a net loss of heat to the environment. Likewise, when temperature drops more energy is needed to maintain the heat requirement of the pigs. As a result, the lower critical temperature or the temperature at which productivity of the animals is adversely affected. Harmon and Lawrence (1995) reported varying critical temperature for hogs at different weights. So to overcome this critical temperature during winter, we provided heating lamps supplying heat equivalent to 50,000 Kcal/h. As a result, the EPH maintained a temperature fluctuation range of 15°C as shown in Figure 2, which was about 3°C lower than the 18°C optimal temperature fluctuation recommended by Wathes et al. (1983) more than two decades ago. Similarly, due to changing worldwide environmental climate and improvement in breeding livestock that could alter body metabolism in relation to the environment, more than a decade later, Harmon and Xin (1995) reported that the critical low temperature for a 109 kg pig is 11.1°C and its critical high temperature was 26.7°C. Therefore, the temperature fluctuation that the animal can tolerate was 15.6°C. Based on the above condition, the temperature fluctuation range in the EPH that we obtained, which is 15°C can already be considered almost adequate to provide comfort for the finishing pigs raised up to 107 kg. On the other hand, the CPH also maintained a steady temperature range of around 11°C. This indicated that it was largely influenced by the outside temperature It is also worth noting that the temperature difference between the upper and lower parts of the pig pens was relatively steady in the EPH at about 1°C (excluding the alley, 6th measured spot). In contrast, the deviation in the CPH was about 2°C. The stable and minimal indoor temperature fluctuation in the EPH was beneficial to the pigs as they are homeothermic and have to maintain a relative constant temperature by adjusting their heat production. Therefore, the more stable the indoor temperature, there will be lesser stress. Geers et al. (1988) reported that if the pigs weight is less than 50 kg, the air temperature within the pig pens has to be maintained at 17°C to 25°C. This jibe with the findings of Harmon and Lawrence (1995) that a 23 kg pig will tolerate a critical temperature of not less than 21°C. Our design for the EPH was able to maintain a narrow indoor temperature range within the pig pens at 17°C to 23°C, which was more than enough to protect the animals from going beyond its critical temperature. However, when the animal has attained its maximum growth potential, the day and night temperature variation can be increased as long as it is within the allowed range. This theory was satisfied as the EPH showed almost no temperature fluctuation, whilst the CPH displayed a steep inclination between the upper and lower altitudes of the pens. This can be interpreted as a large influence of outdoor temperature due to poorly regulated airflow. Consequently, our EPH design satisfied the temperature range of 15.5 to 20°C that guarantees faster growth because of lesser heat loss and more feed intake of pigs as claimed by MWPS-8 (1988).

The airflow velocity of the EPH during winter is shown in Table 2. The inflowing air was slightly warmed near the ceiling and moved downwards to both sides of the walls of the pens then towards the alley. Due to the minimum ventilation by the air fan duct attached to one side of the wall, the velocity of the inflowing air was 0.59 to 0.78 m/s and the average air velocity in the lower area of the stall was 0.06 m/s. This indicates that there was no stagnant air from the different sites of measurement as shown in Figure 2 wherein there is continuous and uniform distribution of airflow. Our data on air velocity in the lower area obtained by our EPH design is almost similar that obtained by Grub et al. (1974) which is at least 0.08 m/s. He further claimed that this air velocity rating was already capable of liberating toxic gases from pig houses. Therefore, the 0.06 m/s air velocity which we obtained considering its closeness to 0.08 m/s may also be sufficient to liberate obnoxious gases which could improve the growth rate of the pigs and further demonstrated by a 6 kg advantage in bodyweight of the pigs during winter time. Moreover, Harmon and Lawrence (1995) reported that an inflowing air velocity of less than 0.15 m/s can result to stress on small pigs and at less than 0.3 m/s can be harmful to large pigs during winter. Unfortunately, no air movement could be measured (0.00 m/s) in almost the lower parts of the front, middle and rear section of CPH as shown in Table 2. This is attributed to the fact that there was negligible amount of airflow in the air slot, which resulted to the adverse effect or inferior growth rate of the pigs reared under CPH.

Based on the above observations, the environmental conditions of pigs reared under CPH are extremely poor since there is no airflow in the lower portion of the pen, which is the critical breathing space of the pig. MacDonald (2005) reported the importance of distribution and velocity are very important considerations in a ventilation system. He further established that the minimum ventilation rate for a 30-kg pig is 650 ft³/m. Apparently, adjusting air slot to improve airflow and subsequently air velocity in the pens was demonstrated in this study.

An increasing indoor temperature can significantly result to higher concentration of NH₃ as demonstrated in the correlation studies of Choi et al. (2005) between air pollutants and thermal environment factors in a confined pig house during winter. The periodic measurements of indoor NH₃ emission during winter in both the EPH and CPH is shown in Figure 3. A maximum concentration of 19.7 mg/L was evident in the CPH whilst an expectedly lower level at 9.6 mg/L was recorded in the EPH. Both indoor emissions were lower than the allowed concentration of 20 mg/L (MWPS 32, 1990). However, there was a great variation in NH₃ emission between summer and winter in the EPH (16 vs 9.6 mg/L) whilst that in the CPH was minimal (17 vs 19.7 mg/L). An important observation in the present study, was that the NH₃ concentration during summer was lower compared to winter in the CPH. Whether variation in metabolic production of NH₃ by the animals due to season will be another subject of investigation in the future so that no adjustment would be done in the design of the EPH. Another investigation that will be done is whether degree of coldness or hotness of air can affect dilution or dissipation rate of NH₃. Variations on the design and dimension of the CPH to include width of air duct will also be done to confirm the dilution potential of air inflow on the NH₃ gas. Another relevant finding was the minimum ventilation rate during winter resulted to an NH₃ level in the EPH, which is 49% lower than the NH₃ emission from the CPH. Starting from February 4, the pigs in our study were already 60 kg and this is the peak of their metabolism rate, that can also result to an increase in emission of toxic gases. Proper ventilation during this period is therefore necessary. Barker et al. (1986) reported that NH₃ concentration levels of pig houses with efficient liquefied excretion disposal systems were 10 to 20 mg/L. The EPH in the present study provided a better environment for the pigs as NH₃ level was low.

As shown in Figure 4 pigs reared in the EPH were heavier by 6 kg than those pigs reared in the CPH during winter. In Table 4, the pigs in the EPH also consumed a significantly higher amount of feeds resulting likewise to a better weight gain than those pigs raised in the CPH. A report of a study at the University of Missouri as cited by Harmon and Lawrence (1995) reported a marked improvement in pig performance reared in enclosed confinement facilities during winter which is in contrast to our result. Pigs raised in the EPH had better performance during summer. Apparently, this is attributed to the humid summer condition in Korea, however, evidence on its direct role should be the subject of investigation in the future. Finally, it was well demonstrated in the present study that air flow dynamics was better in EPH because of the critical role of the inlet (Figure 8) as compared to the CPH without the inlet (Figure 9).

Ventilation system during summer should be designed to maintain heat balance in contrast to winter ventilation wherein moisture balance should be maintained. Likewise, during summer an adequate heat removal by the ventilation system is crucial as suggested by Schauberger et al. (2000). The benefit of the ventilation system of the EPH in maintaining homeothermic heat was demonstrated in this study when it recorded a minimal indoor temperature fluctuation range of only 7°C as shown in Figure 5. This minimal variation was consistent when taken from the different measuring points in all the individual pig pens. Although the outside temperature varied greatly from 23 to 30.5°C, it had a lesser deviation when compared to the EPH thus minimizing stress for growing pigs. On the other hand, the CPH displayed a sharp increase and decline in temperature and almost approximated the uptrends and downtrends of the outdoor temperature. Moreover, it had a higher difference than that of The EPH. This dynamics on the indoor temperature of the CPH during summer can result to heat stress and adversely affect growth performance of animals. During the past, EPH was not popular because of the difficulty to match indoor temperature and body heat emission of pigs to achieve heat balance. The EPH design in the present study was equipped with the needed materials and proper orientation of the ventilation system, as shown in Figure 1. The comfort of the animals was expected as the indoor temperature fluctuation during summer was reduced with a range of 23.8 to 27.5°C only. This observation was based on the report of Spillman and Hinkle (1971) that when a 70-kg pig is reared at an indoor temperature of 22 to 27°C, no heat loss is generated resulting to better growth efficiency which is reflected in the subsequent table of the present study.

A high velocity during summer can make pigs comfortable due to reduced temperature. Our design for EPH in the present study is appropriate for Korean climate as it was capable of attaining a maximum ventilation level of 240,000 m³/h. The air speed distribution from the walls of both sides of the pens and consequently emitted by the air fan duct is presented in Table 3. The velocity of the air that enters from the air slots on both sides of the stall was slightly faster at the center of the stall but generally stable and uniformly evident around the lower areas of the pens (Figure 10). The airflow direction and pattern is presented to serve as a guide on what adverse or beneficial effects it can bring to the animals. This is because varying speeds can reduce indoor temperature and at the same time create cold stress for the animals when it passes through its skin. Therefore, there should be a ideal matching between indoor temperature and air speed. In our EPH design, our recorded air velocity at 0.23 m/s was able to satisfy the optimum air velocity in the confinement area as well as breathing space of the pigs (nose level) set by Sainsbury (1995) of 0.13 to 0.18 m/s compared to our record which is 0.23 m/s. It also complied with the minimum air velocity set by Grub et al. (1974) at 0.08 m/s to effectively expel toxic gases from the pig house.

Conversely, just like the winter setting, there was practically no air movement again in the CPH, particularly in the lower area as indicated by their air speed and airflow direction (Table 3, Figure 11). Ironically, there was faster air speed in the alley, which is not within the direct breathing space for the growing-finishing hogs and is therefore a useless airflow (Table 3). Therefore, Korean livestock farmers who are adopting the CPH, should recognize that this defect in their indoor airflow speed can affect growth performance of their animals.

Although the speed of air that entered through the air slots was lower than what is deemed optional by MWPS 32 (1990), which was 5 m/s for proper ventilation, it was still effective as air flow was rotating throughout the pig pens for both the EPH and CPH (Figures 10 and 11). During summer, the airflow characteristics in the EPH is described such that it proceeded downward after gaining entry into the left air slot. The downward trend continues until it reaches the slat level then bounced back following a counterclockwise motion. After it rises to a height of around 2 meters it returns back to the slat level then follows the same counterclockwise motion or a part of the airflow current can move to the right side of the building and converge with an airflow pattern that is maintaining a circular pattern in a clockwise manner. This is made possible due to the entry of the airflow from the opposite direction. The maximum height level attained is also the same with the left portion of the building and therefore assumed a balanced symmetry of circles in perpetual motion. On the other hand, activation of airflow direction of the CPH during summer originated from the pit exhaust fan of both sides of the building and converged towards the alley after passing the breathing space of the pigs. The airflow of the left portion of the building follows a counterclockwise motion with a maximum height of 2 meters whilst that of the right portion of the building assumed a counterclockwise flow. We also noted that these airflow directions and patterns are similar with those in the winter setting, which can be influenced by the housing systems particularly the dimensions. The only difference detected was that the faster airspeed was very distinct during summer. Therefore, airspeed can be season dependent relative to different housing systems. Presently, there is a dearth of information on the airflow characteristics in pig houses and this exhaustive description of the airflow direction and pattern which was made possible due to pioneering identification of the 27 measuring spots in the two types of pig houses will be a useful information to livestock farmers in formulating strategies for prevention of respiratory problems. This is very important as the higher the indoor temperature in pig houses, the greater will be the influence of air velocity on the living comfort and growth of pigs.

The present study noted that even when the CPH was exposed to outside air, toxic gases within the living quarters were not expelled effectively and showed similar levels to that of the EPH. Usually, a low level of air exchange will result to a higher humidity and consequently higher concentration of noxious gases such as NH₃ and CO₂. Accordingly, minor modification preferably on the location or the width of the EPH to maximize air inflow to air outflow will be tested. Furthermore, another intensive study in the EPH, will be done with emphasis on the air flow current dynamics or expulsion of gas even if they are not concentrated on both the upper and lower parts of the pens.

The comparative concentration levels of NH₃ in both the EPH and CPH during summer is shown in Figure 6. In the CPH, concentration level was stable at less than 17 mg/L towards the market age of the pigs while the EPH was less concentrated and more stable at 16 mg/L, both figures were lower than the tolerable level of 20 mg/L (MWPS 32, 1990). The liquid manure pit system of both houses was also a contributory factor in reducing the NH₃ emission because of the reaction: NH₃+H₂O = NH₃OH, thus diluting ammonia to ammonia hydroxide which is a less obnoxious compound. It is advised further that manure should be scraped and not left behind on the slats as a higher emission level of NH₃ is expected when they become dry. Based on this result, we can conclude that this additional EPH method of modifying indoor environment will enable its widescale adoption by Korean livestock farmers.

As shown in Figure 7, Table 4, the final weight of the pigs reared in the EPH were heavier by 7 kg compared to the pigs reared under the CPH. Consequently, there was a significant gain in weight and feed intake of pigs raised in the EPH over those pigs reared in the CPH. This is an indication that the pigs in the EPH were afforded a better environment and was translated into improved performance in their growth parameters. This also implied that there was lesser stress on their part which is an indication of an appropriate environmental condition. The indoor temperature, which was not largely influenced by the outdoor temperature in the EPH was also a major contributory factor. This is because when temperature drops, more energy is needed to satisfy the heat requirement of the animal. Although feed intake is increased during this period, its maintenance requirement will also increase faster leaving less energy for weight gain and therefore the growth of the animal is adversely affected. Subsequently, to supply the needed energy to sustain normal growth, an effective temperature for a homeothermic animal like the pig is needed. And this is possible if the thermoneutral zone (TNZ) of the pig is maintained. In the study, there was a narrower fluctuation of the temperature in the EPH than the CPH, therefore, better TNZ was attained by the pigs raised under forced ventilation or EPH.