Protection against Air Pollution

CHIANG MAI BURNING SEASON

----- [2024 UPDATE] -----

This webpage is a short summary of the composition of air pollution in Chiang Mai and Northern Thailand. Further information and references can be found in 'Comprehensive Review of the Annual Haze Episode in Northern Thailand (Pirard & Charoenpanwutikul, 2023)'.

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Previous Page: Official Actions on Air Pollution

FAQ

Physical

PREVENTION OF HEALTH RISKS

Prevention is only possible through mechanical air filtering

Outdoor, the use of masks, preferably N95 rating is recommended

Indoor, the use of air purifiers or positive pressure systems

with HEPA filters are recommended

Outdoor techniques for mass filtering show no evidence to work

Seasonal air pollution is not going to disappear anytime soon. While some years (2003, 2011, 2022, 2025) had very mild pollution, most years have several days of extreme air pollution and in the worst cases (2007, 2010, 2023), several weeks when some form of air filtering becomes essential. Three approaches are used to reduce locally the amount of air pollution. At a personal level, the use of masks and respirators; at a room or building level, the use of large filtering devices and outdoor techniques.

Filtering, as home devices or masks, are not just simple sieves as it was often implied during the COVID pandemic by some groups of people. Several physical proceses (sieving & impaction, electrostatic trapping, Brownian motion, adsorption, ...) are at work in an air filter and makes them efficient over a large range of particulate size. For most filters, the lowest filtering efficiency is reached at ~300 nm (0.0003 mm). For larger particle size, many filters are close to 100% efficiency and the same applies to some extent for smaller particulates.

Different scales are used to measure the efficiency of a filter, among them the Minimum Efficiency Reporting Value (MERV). It ranges from 1 to 20, with MERV 8-18 is a typical A/C filter (low EPA), the commonly used HEPA (i.e. car A/C filter) is MERV 17-18. With the exception of some specific professional filters, all these filtering membranes have their weakest point around 300 nm. MERV is applicable to all appliance and filtering systems. For air purifiers, modified A/C and HVAC, the Clean Air Delivery Rate (CADR) in volume per time (i.e. m3/minute) is another important parameter that express how quickly a room can be replaced with a selected quality of filtered air.

Figure 23: Comparison of Northern Thailand haze distribution (mass: red; particulate number: dotted line) with other common pollution and micro-organisms. The range displayed by filters here is partly misleading as all filters work for the whole interval of particulate size but with different efficiency.

.Personal Protective Equipment

When a PM-based AQI reached dangerously high levels, a mask with a high filtering efficiency is strongly recommended. Unless you have specific requirements (no negative pressure, high permeability, psychological effects, other medical recommendation), a mask of N95 or equivalent rating is appropriate. Outflow valves on masks are also important as it does not get soggy due to breath humidity, reducing the risk of rash and bacterial infection. It also prevents the accumulation of CO2 in the mask and minimally improves the efficacy of the respirator by maintaining negative or neutral pressure.

Masks are not entirely beneficial. The increase in humidity, inhaled temperature, lower oxygen intake and inhalation resistance have minor repercussions on blood glucose level, muscular abilities, cardiovascular efficiency and mental skills. It is negligible for a normal effort but during intense physical exercise, it can be significant, increasing heart rate and reaching the maximum oxygen debt (the amount of oxygen required for the exercise) in a shorter time. Some individuals are also psychologically affected when wearing masks with feelings of discomfort, anxiety, claustrophobia, etc. People wearing glasses can be affected by fogging due to increased humidity.

Individuals with respiratory and/or cardio-vascular issues should consult a doctor before wearing a N95 or above respiratory mask. It is possible that such masks might cause more issues than breathing polluted air. Children and people with facial hair should wear a well-fitted mask. Leaks due to a beard or improper size will make a mask not filtering properly while making breathing more difficult.

Commercial masks (no rating)

A lot of masks are available for sale for a variety of purposes and do not follow any filtration standard. These are made of all kinds of materials and various weave and have very different filtering abilities. While some masks will filter 70-80% of particulate matter, it can also be as low as 10%. Surveys also show that some of these unrated masks are equivalent to a N99 but these are exceptions. The main issue with these masks is the lack of reliability. Since there is no standard to be enforced, filtering efficiency can only be obtained from surveys and possibly reliable brands.

It is however worth noting here that some standard-rated masks from some companies fail to reach the filtering efficiency they are suppose to achieve, so a standard is not always a guarantee of efficient filtration either.

Surgical masks (EN14683, ASTMF2100, YY0469)

These masks are traditionally used in the medical environment. They are made of a random mesh of polypropylene fibers and surprisingly, contrary to popular opinion, these materials are as effective as N95-rated fabrics in filtering particulate matter. However, since these are not negative pressure equipment, it allows airflow on the side, limiting the practical efficiency to 65-80%.

A common medical practice in infectious environments is to tape the sides of surgical masks to seal off openings and doubling masks. In such cases, the filtering efficiency is relatively high, reaching 95 to 99.5% of filtering. Single use 'surgical-like' masks with no rating have to be considered in the above category of commercial masks.

Figure 24: Electron microscope imaging of various masks at different magnifications. (a) cotton flannel masks with disorganised layer (b) Woven polyester mesh with very organised layer (c) Cross-section through the filtering layer of a N95 mask made of melt-blown polypropylene fibers.

Respirators

Respirators are masks held into a semi-rigid, well-fitting structure and made of polypropylene fibers manufactured in an electrical field, producing electrostatic filtering. These masks range from the most basic air-purifying mask to air-supplied systems. This latter group is not covered here as it is used for very specific application in extremely harmful environments or life-threatening medical conditions.

N95 (also FFP2, KN95, KF94, P2, DS2, PFF2) is a respirator that filters 95% of particulates at 300 nm and most brands are considerably above the rating they are supposed to achieve. These are the most adequate and widespread masks to filter Northern Thailand air pollution. For the purpose of air filtering in Chiang Mai during haze episodes, respirators can be reused until damaged, soiled or causing increased breathing resistance. It is only in medium biosafety settings that N95 and equivalents should be systematically discarded after use.

N99 (also FFP3) is a respirator that filters 99% of particulates at its weakest value but many brands have a practical filtering value higher than that.

N100 is a respirator that filters 99.7% of particulates at its weakest value. These masks (N99 and N100) have increased breathing resistance.

R-rating is similar to N-rated masks but oil-resistant. They have a short-life and have to be disposed after use. These are for technical work in oil-ladden atmosphere and have no use in Chiang Mai pollution.

P-rating is similar to R-rated respirators but oil-proof, giving them a long lifetime. Again, these are of no use in Chiang Mai pollution.

HE: High Efficiency respirators are similar to P100 but require a powered source. It is only for specific applications and persons medically disqualified from negative-pressure masks.

Non-mechanical respirators are masks using cartridges filled with activated carbon or resins to filter toxic chemical compounds from the air. They are not suitable to filter most particulate matter but will remove gases and nanoparticulates. They are of no particular use in Chiang Mai.

Figure 25: Filtering efficiency curve of various masks. The dip in filtering efficiency at 0.3 microns is obvious for all masks and also show that outside that size range, (larger & smaller), many masks have ~100% efficiency. The variability (minimum and maximum measured values) for surgical masks and cloth masks is also provided.

Indoor Filters

Particulate matter enters closed space through advection (open doors & windows) and infiltration (gaps in the structure). A variety of techniques exist to filter the air inside a closed space, including DIY and commercial air purifiers, modified A/C units and positive pressure systems.

Air purifiers are essentially simple fans pulling ambient internal air through a filter and capturing particulate matter present. They are typically fitted with an HEPA filter but commercial devices occasionally present alternative filtering techniques exposed below. The homemade addition of an HEPA filter on an A/C unit has a similar effect but high MERV-rating filtering are not always suitable for some A/C units as it can bring considerable strain and damage to the motor. In recent years, some A/C system have an inbuilt HEPA filter system that can be replaced.

Positive pressure systems pull air from outside, to push it inside a building, creating a higher pressure indoor and preventing polluted air to get in from other openings. Positive pressure systems have the advantage of replacing indoor air with clean air, maintaining environmental level of CO2, radon and other indoor pollutants but comes in direct competition with A/C system since they constantly bring hot (or cold) and humid (or dry) air inside the building. Some positive pressure system are directly hooked on the HVAC house system but this practice is relatively rare in Thailand.

EPA Filters

It stands for Efficient Particulate Air filters and are membranes with a filtering ability from 80 to 99% (MERV11-16). These are often used in system that requires filtering but do not excessively restrict air flow. These filters are the type found in heating, ventilating and air conditions devices and pre-filters for other systems.

HEPA Filters

It stands for High Efficiency Particulate Air filters and made of a random mesh of polypropylene or borosilicate glass fibers. Various ratings (MERV17-18) are available and will stop up to 99.97% of particulate matter. Their relatively high airflow capacity with very good filtering standard make these filters the most suitable to filter Chiang Mai air pollution during the burning season.

ULPA Filters

It stands for Ultra-Low Particulate Air filters and is the level above HEPA. Their rating is MERV19-20, filtering 99.9999% of particles at 120 nanometers and is therefore particularly effective in that low efficiency gap that other filters have. The high filtering capability considerably reduce airflow and limit the volume of air to be treated. Since it requires several pre-filters, it has a higher cost than a standard HEPA, with more frequent changes for a minimal to zero gain in filtering natural background pollution. The typical use of such filtering devices are clean rooms used for medical, chemical and microelectronic applications where air locks, closed-up dedicated suits and strict entering procedures have to be followed for these filters to actually be effective. The promotion of ULPA filters by companies in Chiang Mai is mostly a commercial marketing technique as the gain is minimal for everyday uses.

Adsorbing filters

An entirely different type of filter made of activated carbon, lime, silica gel or zeolites. The very high surface of these porous form of carbon, silica and aluminosilicates captures nanoparticulates and some gaseous contaminants through the physico-chemical process of adsorption. These filters are not suitable without an HEPA filter as they saturate quickly but have their use to trap some volatile components and smells from biomass (or trash) burning. In some specific industrial application, these filters are essential such as the SO2 filtering in Mae Moh Lignite Plant in Lampang that was a very significant health concern in the 90s and before.

Silver Ion Filters

These are filters that inactivate bacteria, fungi and viruses carried by air pollution. Operational conditions (flow rate, exposure) makes these devices largely insufficient to disinfect purified air. Even if these devices were working, their purpose in Northern Thailand pollution is limited since biological organisms are almost absent from haze.

Air Ionization

A device creating a high voltage to ionize molecules in the air. Through electrostatic attraction, ionised molecules can be removed. Such filters are used in professional environments (i.e. Mae Moh Lignite Plant) but the lack of standard in low-cost air purifiers is problematic. The filtering efficiency only applies to the finest particulates with no guarantee on how efficient they are but more importantly, the lack of construction standard can lead to the production of ozone, nitrogen oxides, formaldehydes, etc. from an initially clean air and resulting in being more harmful than beneficial for health, even if the AQI value is lowered.

Ultraviolet purifiers

A device emitting strong UV-C radiating pumped air to damage the DNA of bacteria and viruses passing through. The process is used in some setting such as ultraviolet germicidal irradiation (e.g. water purification) but in practice, like silver ion filters, the exposure to UV of the air passing through such devices is 10 to 100x too short to significantly kill microbes and of little use in Chiang Mai haze.

As it is the case with air ionization, UV-C can produce large amount of ozone and the lack of construction standard is a source of concern.

Outdoor Filtering

This section covers techniques sought to reduce air pollution in an outdoor situation without direct action on emission sources. It includes various use of wet deposition, scaled up mechanical filtering devices and meteorological actions.

Water spraying

Every year, the local government promotes the use of large water canons spraying droplets in the air to catch particulate matter around Thapae Gate and various suburbs. This technique is commonly used and effective at mine sites and dust-producing factories but spraying of water in open spaces with diluted air pollution has been shown to have no effect in an urban environment.

Atomizers & fogging

A similar techniques that relies on wet deposition. It is normally used for its cooling effect when countless tiny water droplets evaporates, absorbing heat. Scientific studies of such technique for catching particulate matter show that it can make things worse on hot and dry days with mild pollution by dissolving and then reprecipitating soluble particulate matter, effectively increasing the PM2.5 by several times. The effect is unclear on a very polluted day but is probably beneficial. However, the effect is relatively local and is probably best used in a fog curtain type of setup.

Giant Air Filters

Another regularly advertised technique by the local government in their fight against air pollution; the concept is to push air through an HEPA filter using a very large fan. A simple calculation in a theoretical static situation (no new polluted air coming from outside) shows that thousands of Airbus-380 engines working at full power for weeks (assuming clean engines) would be required to clean the air over Chiang Mai metropolitan area.

Cloud Seeding

A yearly practice in Thailand using various salts, dry ice, urea and silver iodide released in clouds to initiate precipitation, leading to rainfall. In North-East Thailand, where this technique is routinely applied, scientific scrutiny show mitigated results with either weak statistical evidence or inconclusive effects despite the Royal Rainmaking Project claiming an 89-100% success rate. Cloud seeding to fight against air pollution is attempted in other countries but the specific meteorological conditions required for a successful seeding (high humidity, mildly turbulent conditions, cloud cover, etc.) are rarely met during the burning season making this option unlikely to work.

Another technique pushed forward by the Royal Rainmaking Project in the past couple of years in Bangkok, is the use of dry ice or icy water to cool the upper troposphere and establish a mild temperature gradient in the lower atmosphere. The thermal inversion layer would then be weaker, helping the dispersion of pollutant in a thicker layer of air. At the time of writing, there is no scientific evidence that such technique work and without clear data from the RRP, the whole thing appear to be more of a PR exercise.

Figure 26: Various outdoor techniques to supposedly reduce the amount of particulate matter air pollution