When going outside? Probably very little unless you can overall reduce the pollution producing factors in your surrounding area. As for inside your home, there

When going outside? Probably very little unless you can overall reduce the pollution producing factors in your surrounding area. As for inside your home, there's a few things but the effectiveness will depend on your situation specifically. The US EPA has a good starting point with information. https://www.epa.gov/indoor-air-quality-iaq/indoor-air-quality-apartments https://www.epa.gov/indoor-air-quality-iaq/indoor-air-qualit...