If you have "breathing problems" the only responsible answer is to get fitted for a respirator.
There are two components to dust collection. The most efficient is to use a dust collector directly at the source of the dust generation. In other words use dust collector design for attachment directly to the machine. If you are really referring to air cleaners that purport to clean the total air in the shop, be aware that then are next to worthless and do not remove dust particles until the dust generator is turned off.
Here is some excellent info about air cleaners.
Here are some general rules based on Nagyszalanczy's "Workshop Dust Control" and some of my own experiences and thoughts.
The two most important criteria for an air cleaner are the CFM and the filters. You want a CFM factor that will clean the size of your shop and a filter that removes the particle size that you are concerned about.
To determine the size or required air flow, use this formula: Volume of your shop (Length x width x Height) times Number of air changes per hour (typically 6 - 8) divided by 60. This will give you an answer in Cubic Feet per Minute which is how air cleaners are measured. MOST AIR CLEANER MANUFACTURERS RATE THE CFM OF THE FAN ONLY, but there are losses due to the filters. If you are building your own or if the air cleaner you are purchasing rates only the fan, figure you will lose about 25 - 40% for filtering losses.
As important as the air cleaner size is how and where you mount it. Try to mount at about 8-10 feet above the floor (no lower than 6'or 2/3 of the floor to ceiling distance if less than 8' ceiling). Mount along the longest wall so the intake is approximately 1/3 the distance from the shorter wall. Mount no further than 4-6 inches from the wall.
The exhaust is the largest determiner of the circulation pattern. You are trying to encourage circulation parallel to the floor/ceiling so ceiling mounting is not recommended. Use a smoke stick (or a cigar) to observe and maximize circulation. Use a secondary fan to direct air to the intake if necessary. Also, consider that a standard 24" floor fan moves a lot of air and, in some shops, just positioning it in a doorway with a window or other door open can accomplish as much or more than an air cleaner. It's all in the circulation patterns.
The exhaust is the clean air so that is where you want to position yourself. Do not place the air cleaner over the a dust producer. That will guarantee that the operator will be in direct line between the dust producer and the air cleaner. The operator wants to be in the clean air stream. If the dust has to pass your nose to get to the air cleaner, you get no benefits. If you have an odd shaped shop, two smaller units may be better than one large one.
DO NOT RELY ON A AIR CLEANER TO ACT AS A DUST COLLECTOR. The purpose of and air cleaner is to keep airborne dust in suspension and reduce airborne dust as quickly as possible AFTER THE DUST PRODUCER HAS BEEN TURNED OFF.
Finally, if you are looking for health benefits, you will not find any air cleaner manufacturer that makes health claims because there are few health benefits. CATCHING DUST AT IT'S SOURCE IS THE BEST LONG TERM GOAL. Rick Peters', author of "Controlling Dust in the Workshop", makes the point that spending your money getting the dust at its source is a better investment than trying to capture it after it is already airborne. If the dust is in the air, it's going to be in your nose and lungs too. Robert Witter of Oneida Air Systems has noted that "overhead cleaners can only lower ambient dust levels AFTER THE SOURCE OF EMISSIONS IS SHUT DOWN, and they take several hours to do this. This is why they are not used in industry." The absolute best answer, if health is the primary concern, is to use a NIOSH approved respirator. The dust cleaner will help keep your shop cleaner but have minimal or no health benefits. OSHA takes this position too. They measure the number of particles per a volume and most air cleaners will not satisfy their specs.
By Robert Witter, Oneida Air Systems, Chief Engineer
"Air quality tests show that Source collection (dust collection) can lower the dust loading in a wood shop by a factor of 5 to 20 times over a shop with no or bad dust collection. The goal is to lower the dust level below 5 milligrams per cubic meter(OSHA reg). A good DC system can keep the dust loading down to 1-2 mgM3. This means you inhale the quantity of dust in ten years that might have inhaled in 1 year. A little dust is OK. Overhead cleaners can only lower ambient dust levels after the source of emission is shut down, and they take several hours to do this. This is why they are not used in industry.Cloth filters can accomplish 99.9 % sub-micron filtration. For example, carbon black can be filtered with cloth but at a 2:1 air to cloth ratio. That's allot of filter bags. Good Felt filters 16oz will filter 99.5% of material between 0.2-2.0 micron at 10:1 air to cloth ratio. Cartridges pleat more cloth area into a smaller space, that's all. In the air sample tests i have seen only a negligible % of wood flour is as small as 1-2 microns, so a filter efficiency of 99% @ 1-2 microns is more than sufficient, and ostensibly represents 100% filtration for wood dust."