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This is a campus building in Aalto University where you have a tree about 10 cm away from the window. The Espoo city does not allow the student organisation AYY to cut down the tree. There is a poor air-conditioning in the room so the fan is essential to speed up the airflow. Without the fan, the window becomes smoky and the room very hot.

I need to find filter to the window so I can keep the fan on also during times of heavy pollen times (15x100cm for the above window). The filter must be sturdy because birds get into the room.

I tested a novel idea of using computer fans with polycarbonate plastic thing for a smaller window but the computer fans provide very small airflow in comparison to the large fan.

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Which kind of filters should I use in windows like above? Is it enough to have a cloth pinned to the sides of the window? Or do I need carbon filter? Can I use filters meant for computer fans like the above?

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up vote 2 down vote accepted

I have not seen conclusive data about carbon stopping pollen.Carbon is very good at catching odour within in air and excellent at absorbing chemicals in water (ie chlorine)

I would strongly suggest you use a 5 micron sponge pre filter. This will stop bigger bits of pollen and other larger particles.

Then after the pre filter use a HEPA filter. You can find cheap ones on eBay - The more expensive ones are used to for micro biology or strict particle sanitization - Like in hospitals, data recovery centres and bio chem labs.

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HEPA filters are used in cars for intake cleaning, cabin air, vacuum cleaners and ventilation systems. The prices depend on the brand but essentially are the same as any other HEPA filter. You can build a box with inlet pipe that goes outside. In the box you can put pre filter material like 5 micron, sponges or cotton. Then slot in the hepa filters somehow. They must be air tight to work properly. an extractor fan running the other way (taking air form outside pumping into the box) will do a decent job.

I my self have very bad hay fever and pollen allergies. I use ionisers in my house and my vacuum has HEPA filters on the exhaust. It works really good for me.

This brand might be clear enough proof that carbon on its own wont remove all the particles.

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This site can help you start with DIY solutions for HEPA filters

And this one too

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Are the ionisers you use negative ionisers? The full-blown HEPA device (specs) lacks the negative ionisers that can alleviate asthmatic symptoms such as itching and shortness of breath by this so a better set-up could be to buy a cleaner/cooling/filtering system and small negative ioniser towers such as here to alleviate allergic symptoms? – hhh May 7 at 10:55
I moved a part of this question here: the answer seems to be yes, negative ionisers are effective to alleviate allergic symptoms against pollen particularly when combined with proper ventilation system, right? – hhh May 7 at 11:06
Well yes. The negative ions bind to the pollen and make it fall to the ground. You need to vacuum frequently with a vacuum that has a HEPA filter too or use a HEPA filter on the intake to reduce pollen intake. Then use ionisers to bind to the "remaining" polen and dust – ppumkin May 9 at 9:00

You'd be better off with a box fan as it has a large, square area that will support your filter media properly. It will need to be powerful enough to develop airflow despite the next addition.

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Not sure if you can find it, but we have here available electrostatic filter media by the roll. Cut several pieces large enough to cover the inlet side of the box fan. Mount one to the grill, keep an eye on it for pollen buildup, swap in a spare and wash the clogged one.

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It's not as efficient as a HEPA filter system, but then the cost of the filter material isn't going to put you in the poor house like the cost of expensive HEPA microfilters that will quickly clog even though you keep the outer prefilter cleaned will.

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Box fan with a high-quality filter is a superb idea but unfortunately my area of Finland lacks this kind of items on the market so decided to go with air-conditioning-system: it filters the air, adjust the temperatue and increases the airflow. I like it but I am going to try this "electrostatic filter media" in other apartment -- why is it called "electrostatic"? – hhh May 30 '13 at 20:18
It is made of two different types of polymer filter material. Airflow through it causes a static charge to build up which causes dust, pollen, etc. to stick to it. – Fiasco Labs May 31 '13 at 0:49
+1 thank you, makes sense. Will try it! – hhh May 31 '13 at 10:45

Get an air-conditioning system. I got it because my room has no output air channel so adding an extra output air-channel through the air-conditioning system fixed the problem of smoky windows. It filters the air and it is pretty easy to install: it takes air 1 block inside and 1 block moving outside so the pipe just to the window for the output channel.

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The downside of air-conditioning system is the energy consumption. Smallest/cheapest unit I could find was 250EUR "ilmastointilaite" unit from Bauhaus with 2k W/h energy consumption (only one offering 5 years' warranty hence chose this model) so it costs about 15 cents in Finland to run one hour with the full mode but 15 minutes running does very good job in cleaning the air and making the room comfortable to sleep and work. So it is relatively cheap to filter the air unless much higher electricity prices and/or far larger/higher apartment.

I don't like to sleep with it on because of the noise. So I created an output air channel to the top of the window in PMMA plastic ("kelmumuovi") and the hot output channel to the bottom for the machine when it is operating: this means that the room air will go from the top when the machine is not on and when it is on the air will go away from the below hole. This saves energy and keeps the cold air inside the room and hot air outside because the special PMMA has air between the plastic strengthening and air is an excellent insulator.

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