It's not consciously-imperceptible levels, it's just not a particularly salient sensation. I notice the headache, then I notice the smell, and my brain helpfully informs me "hey, you've been smelling that for, like, the past 30 seconds". If the air's really cold, sometimes I don't smell anything until I warm up the air I'm breathing – but that's not imperceptible levels, that's just my nose not working in the cold. If I'm paying attention to what I'm smelling, I can stop breathing before I get a lungful, and that usually avoids any negative effects. But there are a lot of smells in a city, and I haven't developed reflexes for marijuana like I have for tobacco.

When I'm exposed to higher concentrations, either I get a stronger headache, or the headache goes away and I get light-headed. I haven't died yet, so I don't think it's a medical emergency of any kind.

I don't even know if it's because of the marijuana. Perhaps the breeze is concentrating a load of (e.g.) carbon monoxide in certain places, and it just happens to also concentrate marijuana scent… but I think it's probably the marijuana. https://www.durham.ac.uk/research/current/research-news/thre... has some possible explanations:

> The chemicals that activate the smell signals in our brain (called odorants) can sometimes irritate our sinuses. Smoke, perfume and chlorine are some of the most common odorants that cause irritation.

> When [the trigeminal] pathway is stimulated it causes inflammation because it detects a threat that only the immune system can sort out. This, too, can lead to a headache. Chemical smells such as formaldehyde, certain cleaning products and cigarette smoke are all known to act directly on the trigeminal pathway.

Some cannabinoids are known to act on the trigeminal pathway, and I don't usually get headaches from smoke, so it might be that… but I haven't run any experiments, so who knows?