"I'm due any day now, and my pediatrician, whom I think is quite sensible about most things, recommends that I don't take the baby outside at all for six weeks because of the high level of pollutants and allergens in my area. My husband has seasonal allergies, so it's something I'm aware of. This summer hasn't been a bad summer at all. No code red days, and Hubby hasn't resorted to his allergy meds nearly as often as usual. Six weeks seems like a long time to keep a baby with a normal immune system indoors. It seems like a very, very long time to keep me indoors. Can this be one of those situations where I take the pediatrician's advice under advisement, and then do what feels best to me?"
Do you live in Mexico City*? If not, there's no reason to assume that your baby is going to have abnormal problems with weather conditions or pollutants. If your area is having alarm days, obviously follow the recommendations to stay inside during peak hours, but going out in the early morning and in the evening should be fine. I'm assuming you'll be paying attention to the baby while you're out and not just sending your monkey butler out to walk the child, so if the baby starts to have any problems you can go inside again right away.
All the warnings against going out I've heard have been based on not taking your child to enclosed areas with a bunch of people with their accompanying bunch of germs. If you're breastfeeding you don't have to worry so much, because your milk passes on immunities to the baby (whose own immune system isn't really in full force for a few weeks). The best way to manage this exposure is to stick to places where there aren't as many people or they're all spread out, and not tight or crowded groupings. So huge mega-grocery store or short walks around the block--yes. Crowded subways or dorm parties--no.
You can also keep your baby more protected by carrying the baby in a body carrier (sling, pouch, wrap, etc.) close to you. People seem to be less inclined to walk up to and try to touch a baby carried close to a parent than one in a stroller. Also, you can block anyone with your hands if the baby is right there. And they're not exposed to the world, but snuggled right up in your airspace.
I think you should plan on being inside for at least a few days, if not a couple of weeks, just to give yourself a chance to rest and reenergize. (In many cultures, new mothers and newborns stay inside and everyone else brings them everything they need for 40 days. So from a resting-up and bonding standpoint staying inside makes a lot of sense, assuming there are plenty of other adults there to take care of you and keep you company. But that's a huuuuge assumption.) But then you're not going to be going out on any all-day hiking trips that soon anyway, so just do what your energy level dictates. As long as the baby's with you and you're watching his or her cues, you'll be fine.
*Don't any of you chilangos get mad at me for talking smack about El Districto. You know it's super-polluted. When I lived there in '95 we used to joke that they should just chop off the last two places of the ozone index, like they did with the peso a few years previous to that. As far as I'm concerned, the pollution and traffic are the city's only flaws. Lindo y querido.