Found one scientific write up regarding indoor allergens like mold/fungi, cat allergen (i guess uncommon in SG due to HDB rules), mites, pollen (not all would be in your house, esp for the last one as we do not usually have pollen issues) and how ACH/HEPA filtration reduced the indoor concentrations.
Source is a PDF.....can click on it, it is safe.
https://www.jacionline.org/article/0091-6749(88)90980-3/pdf
I think maybe i shall post it out for readers' ease.
INDOOR ALLERGEN AND IRRlTANT CONCENTRATIONS
The quantity of allergen in the air of the indoor environment is related to the rate of introduction of that allergen from outdoors, as in the case of pollen and some mold spores, or production withm the house, as in the case of other species of mold spores and allergens derived from animals and insects. A second variable is the rate the allergen settles from the air. Depending on the size of the particle with which these antigens are associated, rate of spontaneous settling will vary greatly; generally, the amount of antigen associated with surfaces either due to direct deposition or to settling will far exceed that in the ;
Pollen allergens Most pollen is >15 pm in diameter and settles rapidly from still air. The rate of settling for a 20 um particle has been calculated to be 2.4 ft/min.” This rate has been confirmed in actual observations of ragweed pollen showing almost complete clearing from still air within 5 minutes.24 It is because of this rapid settling indoors and isolation from outdoor sources allowed by air conditioning, when it is present, that low indoor counts are observed even in the absence of airfiltering devices. There is evidence, at least during the ragweed season, that there are ragweed allergens carried on particles and perhaps vapors far smaller than pollen grains. 25,26 These may arise from other ragweed plant parts, particularly the inflorescence,” or by leaching of allergens from the pollen grains after they have been released and fallen on other surfaces. 26 Thus far, the occurrence of these allergenic particles and aerosols has been assessed only in the outdoor air, and their importance in indoor air is not known.
Mites
The principal allergen of the house dust mite, Dermatophagoides pteronyssinus, is associated with fecal pellets >lO wrn in diameter.*’ The settling rate for particles of this size is estimated to be 0.6 ftl min.2’ As might be anticipated, particles of this size are largely airborne as a result of domestic cleaning and changing the bedclothes; 15 to 35 minutes thereafter, 96% of the suspended mite allergen has settled out of the air.” Direct sampling in the air of 16 undisturbed rooms at a rate of 17 L/min for 2 to 8 hours did not reveal detectable quantities (BO.3 ng Der p I) of mite antigen, nor could this mite allergen be found when overnight collection at 3.5 L/min was performed at the level of the head of a sleeping person.28 Other studies have detected the presence of mite allergens in the undisturbed air of homes and apartments.25 Although comparison of different types of allergens is hazardous, the quantity of mite allergen in the air (2661 pg/m3) was markedly lower than the quantity of cat allergen in houses with one to three cats (53,333 pg/m3) or the amount of guinea pig allergen in houses with one to three guinea pigs (306,500 pg/m3). Sampling data are thus consistent with .a tendency for most mite allergen to settle from the air, which raises a question as to whether significant amounts are in the air except during domestic activities.
Animal allergens
As suggested by the data mentioned above,25 it appears likely that allergens derived from domestic animals remain airborne to a greater extent than those from mites and pollen grains. Sampling of an apartment where cats had resided revealed that 8 1% of the Fe1 d I allergen (Cat 1) was associated with particles >lO km in diameter.29 After simulated domestic activities, however, a notable quantity of allergen was present in the air associated with very small particles (<0.05 pm) that remained airborne with little reduction 2 hours later.23, 25 Rat urinary proteins have been found to be associated with particles in the 5 to 10 km range. Mouse urine proteins were found in appreciable quantities in the air of inner-city apartments.25. 27
Molds
Although during the warmer months indoor mold sampling is often dominated by outdoor fungi,30 the principal fungal spores of indoor origin have consistently been Penicillium, Aspergillus, and yeasts.30-32 All of these produce small spores that might be expected to remain airborne for considerable periods. Sampling in homes in Michigan during the winter revealed isolation rates as high as 14,000 spores /m3, with 15 of 80 samples having >lOOO isolates/m3.30 Asthma has been provoked with inhalation of both Penicillium and Aspergillus spores in quantities that might be inhaled as a result of indoor exposure over a 24-hour period.33 Cockroach Unlike the house dust mite, the allergens of cockroach appear to be contained primarily in the body and shed skin rather than in the feces.34 In a study of aeroallergens in 20 inner-city apartments, the quantity of cockroach allergen detected in the air was 29,450 pg/m3.23
Summary
Allergens can be found in the indoor air. It appears that the rate of spontaneous clearing of the air by settling is higher for pollen and mite allergen, and significantly less for mammalian allergens and perhaps mold spores and cockroach. Although these allergens are to be found in the air, this should not obscure the fact that the airborne load may be very small compared with that on surfaces. For example, a spread on which cats had slept regularly was extracted and yielded 20,000 U of Fe1 d I antigen, a quantity equal to 74 times the amount that could be obtained from washing a cat.” Clearly, with quantities such as this on the surfaces that are readily dispersed by activity, the small amount of allergen available to be removed from the air may be insignificant .
RECOMMENDATIONS
Different allergens are associated with particles of different sizes that vary in their tendency to remain airborne. Thus pollen grains and house dust mire feces predictably fall from still air within a few minutes, whereas allergens derived from animals and mold spores may remain airborne to a greater degree, although even they settle significantly from the air. The amount of allergen in the air, even from domestic animals, is relatively small compared with the huge reservoir existing in the furnishings and carpets of the home. There is no information on the threshold dose of any indoor allergen required to produce symptoms. Furthermore, the relative importance of continuous low-grade exposure as opposed to intermittent exposure to heavy concentrations of allergens is unknown. It is possible the latter is more apt to increase nonspecific bronchial hyperreactivity. The most predictable health benefit will be achieved by eliminating the source of the allergen from the home-be it a pet or a heavily mite-infested sofaor by environmental control measures designed to decrease the exposure to mite allergen.5’. 52 Important measures, when feasible, are to increase ventilation with outdoor air and, where appropriate, to reduce humidity, a measure that decreases both mite16. ” and moldi growth. The available data suggest that the role for freestanding indoor air cleaners is limited. Published reports indicate that most of the prevention of exposure to outdoor allergens during warm months can be obtained by use of air conditioning, with only marginal additional benefit from a central air cleaner. Studies of room-cleaning devices have not produced significant clinical improvement, except with the use of a laminar how of highly cleaned air over the subject’s head during sleep. Significantly, in one study, similar results appeared to be achievable by encasing the mattress in an impermeable covering. This reinforces the feeling that avoidance measures and reduction in the reservoir of allergen should be used first, and aircleaning devices should be considered only if symptoms remain severe despite other avoidance measures and there is reason to believe that a significant load of airborne allergens is present. If the decision is made to use an air cleaner, which type should be obtained? It is clear that the ordinary panel filter unit, often producing a scent, with very low cleaning efficiency is difficult to recommend. Because the designation of a filter as HEPA ensures a high degree of efficiency, it should be sought if a mechanical filter is to be used. Electronic air cleaners are initially somewhat less effective and can rapidly decline in efficiency unless regularI!, maintained.
There does not appear to be any obvious advantage to counterbalance these definite disadvantages of the electronic air cleaners when compared with HEPA filter units. In addition, units of mixed pedigree are available. The best way to compare units is to obtain air output figures, preferably from an independent testing laboratory. With this information, rates of clean air change per hour can be calculated for the area to be serviced. For units in which electrical attraction is used in the collection process, some documentation of the rate of decline in efficiency with use will provide insight into the frequency with which maintenance procedures will be necessary. In summary, the available data on the use of residential air cleaning devices has been reviewed. Absence of adequate data on the clinical relevance of indoor ambient allergen levels and the effect of air cleaning devices on these levels precludes any firm recommendations for their use. It is clear, however, that the use of air cleaning devices in the absence of other forms of environmental control is not sensible
(easier too, coz i know these customers aren't ngeow and won't call you to show them that you have a ACRA biz reg so dont mind so-called "fly by night" pax. In any case i wont be able to fleece them with gas top-ups, simply coz i don't know + dont have any gas top up equipment in any case. LOL! 
