Mould contamination poses a health threat associated with people’s symptoms of asthma, cough, wheezing, and upper respiratory infections. Mould and its metabolites can be harmful to human body, especially toxic metabolites are more harmful. Mould spores and their tissue fragments may be allergens, which can lead to allergies. The mould can cause harm when they enter the body through the respiratory tract and the esophagus and can be parasitic on the surface of the human body. About 6 to 15 percent of people are allergic to mould, the pathogenic effects are more severe in allergic people [11] .

Epidemiological evidence suggests that mould increases the risk respiratory diseases such as allergic rhinitis and asthma [12] . Some of the more toxic moulds may cause serious lung lesions or even death. Mould contamination also can increase the incidence of rare diseases, such as alveolitis, chronic and anaphylactic sinusitis. In addition, indoor mould can also have adverse effects on people’s psychology and affect people’s work efficiency [13] .

The damage mould work on buildings also can’t be ignored. In 1996, The Third Assessment Report published by the German research institute claimed that the annual economic losses caused by mould were more than 200 million Euros [14] . From 1999 to 2000, the number of building insurance complaints in Texas increased 13 times because of mould contamination [15] .

Mould can form a lot of ugly stains on the surface of buildings or objects, affects aesthetics. There are many kinds of mould, and the corresponding mycelia comes in different colors, including yellow, green, brown, grey, black and white [16] . Not only does mould contamination may cause corrosion and bulging on stucco coating or paint of wall surface, but also it causing the decaying of wood components, even damaging the structure of the structure, resulting in the expansion or extension of concrete structure pores and cracks [17] [18] . It’s particularly serious that the influence of mould contamination in the buildings with high requirements for interior decoration.

In the steady-state model, it is considered that the mould begins to grow when the environmental temperature and humidity exceeds the critical condition, i.e. critical growth isopleth given by the model. In the prediction of mould growth, the temperature and humidity are usually using the average value over a certain period of time. The steady-state model is a simple predict method but fails to assess the extent of mould contamination, wherefore it is used for prevention rather than assessment. In addition, steady-state models mainly focus on the effect of temperature and humidity on mould, while ignoring the effects of nutrient.

The following assessment criteria for mould risk has been adapted by ASHARE [25] : With the average surface temperature from 5˚C to 40˚C, the average surface relative humidity should less than 80% within 30 days, less than 98% within 7 days and less than 100% within 24 hours.

Hens [26] found that Aspergillus versicolor has the lowest critical growth isopleth among various fungi on the inner surface of building. After laboratory studies, the critical relative humidity of Aspergillus versicolor mould was obtained, based on a quadratic function of temperature correspond to the long- term risk, and a logarithmic function of the number of days correspond to the short-term risk. Clarke and Rowan [27] divided indoor common mould into six categories, each category has their own isopleth for temperature and relative

humidity, and specific examples of mould in each category also are included (Figure 2): Highly xerophilic (Aspergillus repens); xerophilic (Aspergillus versicolor); Moderately xerophilic (Penicillium chysogenum); Moderately hydrophilic (Cladosporium sphaerospermum); hydrophilic (Ulocladium consortiale); Highly hydrophilic (Stachybotrys atra). According to a large number of experiments, the temperature and humidity isopleths for six mould categories were got.

In transient model, surrounding temperature and humidity are fluctuating, and mould growth is regarded as the accumulation process. After a period of time spore needs in favorable condition, it will germinate. Transient model can be used as a mould risk assessment for existing buildings; it is credible but more complex and needs to be calculated hourly. Moreover, the transient model used for quantitatively evaluates the extent of mould contamination, and different models have different evaluation indicators.

Adan [28] put mixed fungi under a cycle environment between high-relative humidity and low-relative humidity. The result showed that mould will grow when the ratio of time in a high relative-humidity environment to total time is higher than 0.5. Further, there is a non-linear relationship between the ratio and mould growth extent. Hukka and Viitanen [29] [30] tested common building materials, put forward the VTT model, and used mould index M as mould assessment indicator, which is a piecewise function to express the difference in favorable and unfavorable conditions. Sedlbauer [14] classified common moulds and common materials in the building, and established Biohygrothermal model, used the mycelium growth rate (mm/year) as the mould risk evaluation indicator. Moon [20] [31] figured that uncertainties can occur at whole building lifecycle and lead to mould growth. He developed a mould performance indicator that quantifies all building parameters and environmental parameters what may affect mould growth, and calculated the mould risk for random sampling results over these intervals of parameter values. Finally, the probabilistic mould growth risk assessment was obtained. Comparing with the field-measured results, Moon’s assessment results of two buildings in Atlanta and Hawaii are consistent with the actual situation.

Indoor environment and people activities provide favorable condition to mould growth. Heat and moisture transfer in the building envelope is an important factor affect the indoor environment, the process can described below [32] :

( ρ m c p , m + ω c p , l ) ∂ T ∂ t = ∇ ( ( λ + h l v δ p φ d p s d t ) ∇ T + h l v δ p p s ∇ φ ) (1)

ζ ∂ φ ∂ t = ∇ ( ( δ p φ d p s d T + K l ρ l R D ln ( φ ) ) ∇ T + ( δ p p s + K l ρ l R D T φ ) ∇ φ ) . (2)

The heat and moisture transfer are simultaneous and coupled each other. When the relative humidity of wall inside or surface reaches the critical condition, there will be a great mould risk. In actual situation, the transfer process is more complex because of various effect factors, such as water leakage and air infiltration caused by wall crack, types of building materials and wall construction, the time-domain characteristics of climatic geography, and the different building functions and live requirements, all of these may change the position of mould growth and reproduction [33] . In addition, the moisture accumulation inside the wall caused by the poor construction quality is easy to cause greater mould risk in a short time after construction (Figure 3).

It is easy to form a thermal bridge since the heat transfer performance of doors, windows, columns, beams and other structures is different from the wall materials. It is easier to reach the critical condition and form mould contamination at the interface between the thermal bridge and the surrounding wall [34] . The corner is also easier to reach critical conditions because of its larger contact with the outdoors.

The risk of mould at the bottom and top of the building is incredible high. The ground floors and walls are highly susceptible to moisture transfer from the soil because of the capillary force, resulting in increased moisture content. There is higher risk of mould contamination in the roof because of the possibility of rainwater leakage and the greater exposure to outdoor environment, combined with the impact of habitat activities [35] .

The air exchange between indoor and outdoor can also lead to mould contamination. In the high-humidity climate, if the outdoor high-humidity air entered the room through building envelope, it will directly cause the mould multiplication. Even if the outdoor air doesn’t meet the critical conditions, it is also easy to reach the critical condition when air contact with colder wall or object, such as the condensation in the moist season in China.

The convective heat transfer coefficient and the convective mass transfer coefficient of the surface are closely related to the surrounding air velocity. Therefore, the air flow state also directly affects the heat and moisture process and results in uneven heat and moisture in all parts. It also explains why mould contamination is easy to appear in the corner and shaded area.

The thermal and moisture storage properties of materials also have influence on mould contamination. For the wildly fluctuate temperature and humidity, good heat and moisture storage property have regulating effect on the peaks of indoor air temperature and humidity [36] , and alleviate the mould risk to a certain extent. However, if the relative humidity in the environment is too high, good moisture storage property will allow the material to retain a high moisture content for a longer period of time, which increase the risk of mould contamination.

Resident activities can also contribute to mould contamination. There are a large amount of moisture sources for living activities, such as clean water in bathroom and cooking steam in kitchen, it will rapidly increases the humidity nearby, provides conditions for mould growth and reproduction. More seriously, water vapor may spreads to other indoor spaces and affects the nearby building environment.

In conclusion, the mechanism of heat and moisture transfer in the building environment is very complicated; especially in the areas with high humidity where the critical conditions are easily reach in the indoor environment. Therefore, targeted measures are needed to prevent mould growth.

Controlling the temperature and relative humidity under critical conditions is the key to prevent mould contamination. Therefore, it is necessary to take into account the whole construction environment and analyze the coupling effect of the air.

When calculating the minimum thermal resistance of the building envelope, full consideration should be given to the effect of moisture transfer, determine the minimum thermal resistance of the building envelope for preventing mould growth [37] . It is necessary to make good water-proof and moisture-proof measures for the areas of moisture source, such as the ground floor and toilet. In order to avoid the crack and leakage in the building envelope, thermal insulation and moisture-proof treatment should be done at the thermal bridge.

Emphasize scientific ventilation. When outdoor humidity is relative higher, it is not allowed to enter the room directly; On the contrary, when outdoor humidity is relative lower, ventilation should be strengthened in order to timely discharge the wet air generated by indoor activities, and reduce the moisture content of the building envelope.

To reduce the concentration of spores in the air, it’s necessary to reduce the indoor exposed soil components, such as reduce number of plants, and make the paint treatment for mud brick. In addition, regularly ventilation also can reduce the concentration of air spores; Materials that have smooth surface can prevent the accumulation of dust, decrease the number of fungi in the material surface and slow down the proliferation speed; Timely cleaning can make the reduction of the nutrition that mould need and the exposure time of mould in favorable environment.